Q.Define the following modes of expressing the concentration of a solution. Which of these modes are independent of temperature and why?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Mass Percentage
Mass Percentage: The Intuition
Imagine you're making lemonade. You mix 50 grams of sugar into 200 grams of water. The total drink weighs 250 grams. Now, if someone asks, "How much of this drink is actually sugar?" — you're not just saying "50 grams." You want to say what fraction of the whole mixture is sugar, scaled to a convenient 100.
That's mass percentage. It answers: "Out of every 100 grams of the mixture, how many grams are this particular component?"
In our lemonade, sugar is 50 g out of 250 g total. That's 25050=0.2 of the whole. Multiply by 100 to get the percentage: 0.2×100=20%. So, 20% of the drink's mass is sugar. If you had 100 g of this lemonade, 20 g of it would be sugar.
The Precise Definition
Mass percentage of component=Total mass of mixtureMass of that component×100%
The formula is simple, but the key is understanding what "total mass" means. It's the sum of masses of all components in the mixture — nothing more, nothing less.
Why It Matters in Chemistry
Mass percentage is one of the most common ways to express concentration — how much of a substance is present in a mixture. You'll see it in:
- Solutions: "10% salt water" means 10 g of salt dissolved in enough water to make 100 g of solution (not 10 g salt + 100 g water — that would be 110 g total, giving only about 9.1%).
- Alloys: "18-karat gold" is 75% gold by mass (18 parts gold out of 24 total parts).
- Food labels: "Fat: 15%" means 15 g of fat per 100 g of the food.
A Common Mistake
Students often think "10% salt solution" means 10 g salt + 100 g water. That's wrong. It means 10 g salt + 90 g water = 100 g total solution. The denominator is total mass, not the mass of the solvent alone.
Step-by-Step Example
Problem: A solution is made by dissolving 25 g of glucose in 175 g of water. Find the mass percentage of glucose.
Step 1: Identify the component you care about — glucose (25 g).
Step 2: Find the total mass of the mixture.
Total mass=25 g (glucose)+175 g (water)=200 g
Step 3: Apply the formula.
Mass percentage of glucose=20025×100%=12.5% …
Why this formula?
Let's break down Mass Percentage from first principles. The goal is to understand why the formula is what it is, not just to memorize it.
1. The Core Idea: "Part of a Whole"
Mass percentage answers a simple question: "If I break a mixture into 100 equal parts by mass, how many of those parts come from a specific component?"
Imagine you have a bowl of fruit salad. The total mass is 500 grams. The apples in it weigh 100 grams.
- The apples are a part of the whole salad.
- The whole salad is the total.
The mass percentage tells you the fraction of the total mass that is apples, but expressed "out of 100" (per cent).
2. The Natural First Step: The Fraction
Before we talk about "percentage," we talk about the fraction of the total:
Fraction of component=Total mass of mixtureMass of component
For the apple example:
500 g100 g=0.2
This means 0.2 (or one-fifth) of the total mass is apples. This is the pure ratio — no scaling yet.
3. Why Multiply by 100?
A fraction like 0.2 is perfectly correct, but it's not intuitive for quick comparison. "Per cent" literally means "per hundred" (from Latin per centum).
To convert a fraction into a "per hundred" number, we multiply by 100:
Percentage=(Fraction)×100
So:
0.2×100=20%
This tells us: "Out of every 100 grams of fruit salad, 20 grams come from apples." That's much easier to visualize.
4. The Final Formula (The "Why" in One Line)
Putting the fraction and the "times 100" together gives the standard formula:
Mass percentage=Total mass of mixtureMass of component×100%
Why does this work?
Because it's just:
- Find the proportion (part ÷ whole).
- Scale that proportion to per hundred (× 100).
5. A Common Exam Trap (and Why It's Wrong)
Sometimes students write: …
Concept: Mass Percentage and Temperature Dependence
Mass percentage (w/w) is defined as:
Mass %=mass of solutionmass of solute×100
Volume percentage (V/V):
Volume %=volume of solutionvolume of solute×100
Mass by volume percentage (w/V):
Mass/Volume %=volume of solution (mL)mass of solute (g)×100
Parts per million (ppm):
ppm=mass of solutionmass of solute×106
Mole fraction (x):
xA=nA+nBnA
Molarity (M):
M=volume of solution (L)moles of solute
Molality (m):
m=mass of solvent (kg)moles of solute
Temperature independence arises when the expression uses mass (not volume), because mass does not change with temperature, while volume expands/contracts.
Temperature-independent modes: …
Concentration modes based on mass (w/w, ppm, mole fraction, molality) are temperature-independent because mass does not change with temperature. Volume-based modes (V/V, w/V, molarity) change with temperature because volume expands or contracts.
The Core Idea: Mass vs. Volume
Temperature affects the volume of a solution — liquids expand when heated and contract when cooled. But mass? Mass stays constant regardless of temperature. So any concentration unit that uses mass in its definition will be temperature-independent, while any unit that uses volume will change when the temperature changes.
Let's examine each mode one by one.
1. w/w (Mass Percentage)
Mass percentage is defined as:
w/w=mass of solutionmass of solute×100
Both numerator and denominator are masses. Since mass does not change with temperature, w/w is temperature-independent.
2. V/V (Volume Percentage)
Volume percentage is:
V/V=volume of solutionvolume of solute×100
Both volumes change with temperature (liquids expand/contract). So V/V is temperature-dependent.
A common mistake is to think V/V is independent because both volumes change "in the same way." But they don't necessarily expand at the same rate — different liquids have different coefficients of thermal expansion. Even if they did, the ratio would still change because the volumes themselves change.
3. w/V (Mass by Volume Percentage)
This is:
w/V=volume of solution (mL)mass of solute (g)×100
The numerator (mass) is temperature-independent, but the denominator (volume) changes with temperature. So w/V is temperature-dependent.
4. ppm (Parts Per Million)
ppm can be expressed in different ways. The most common definition for solutions is:
ppm=mass of solutionmass of solute×106
Since this is a mass/mass ratio, ppm (when defined as mass/mass) is temperature-independent.
Be careful — ppm can also be defined as volume/volume (e.g., ppmV in gases). In that case it would be temperature-dependent. But in solution chemistry, ppm almost always means mass/mass.
5. x (Mole Fraction)
Mole fraction is:
xA=nA+nB+…nA
where n represents number of moles. Number of moles depends on mass and molar mass — both temperature-independent. So mole fraction is temperature-independent.
6. M (Molarity)
Molarity is:
M=volume of solution in litresmoles of solute …
Method: Temperature Dependence Analysis of Concentration Units
This method checks whether a concentration unit depends on volume (which changes with temperature) or only on mass/moles (which are temperature-independent).
Step-by-Step Reasoning
-
Identify which physical quantities change with temperature
- Mass → does not change with temperature
- Number of moles → does not change with temperature
- Volume → changes with temperature (expansion/contraction)
-
Classify each concentration unit
| Mode | Definition | Depends on Volume? | Temperature Independent? |
|---|---|---|---|
| (i) w/w (mass percentage) | mass of solutionmass of solute×100 | ✗ No | ✓ Yes |
| (ii) V/V (volume percentage) | volume of solutionvolume of solute×100 | ✓ Yes | ✗ No |
| (iii) w/V (mass by volume percentage) | volume of solution (mL)mass of solute (g)×100 | ✓ Yes | ✗ No |
| (iv) ppm (parts per million) | mass of solutionmass of solute×106 | ✗ No | ✓ Yes |
| (v) x (mole fraction) | total moles in solutionmoles of solute | ✗ No | ✓ Yes |
| (vi) M (Molarity) | volume of solution (L)moles of solute | ✓ Yes | ✗ No |
Here are the common mistakes students make when tackling this exact question, along with the conceptual fixes to avoid them.
Mistake 1: Confusing which units are temperature-dependent
The Error: Students often say "Molarity is temperature dependent because it uses volume" but then incorrectly state that w/V (mass by volume percentage) is temperature independent.
Why it happens: They memorize "Molarity depends on temperature" but forget that any expression using volume (V) is temperature dependent because volume expands/contracts with heat.
How to avoid:
- Rule of thumb: If the definition contains the word volume (liters, mL), it changes with temperature. If it contains only mass (grams, kg) or moles, it does not.
- Correct classification:
- Temperature independent: w/w, ppm, mole fraction (x), molality (m) — all use mass or moles only.
- Temperature dependent: V/V, w/V, Molarity (M) — all use volume.
Mistake 2: Forgetting that ppm is just a scaled version of w/w
The Error: Students treat ppm as a completely separate concept and fail to realize it is mass/mass (or sometimes volume/volume) multiplied by 106.
Why it happens: The term "parts per million" sounds abstract, so students memorize it as a formula without linking it to w/w.
How to avoid:
- Remember: ppm = mass of solutionmass of solute×106.
- Since it is a ratio of masses, it is temperature independent for the same reason w/w is.
Mistake 3: Writing the formula for molality (m) incorrectly
The Error: Students write m=volume of solvent in Lmoles of solute (confusing it with molarity).
Why it happens: Both start with "m" and both involve moles. The difference is the denominator.
How to avoid:
- Mnemonic: Molarity = Moles per Liter of solution (volume). molality = moles per kilogram of solvent (mass).
- Key exam point: Molality uses mass of solvent (not solution), so it is temperature independent.
Mistake 4: Forgetting to specify "solvent" vs "solution" in definitions
The Error: In definitions, students write "mass of solute / mass of solution" for w/w but then write "moles of solute / mass of solution" for molality.
Why it happens: Careless reading of the denominator.
How to avoid:
- Memorize the exact denominators:
- w/w: mass of solution
- m (molality): mass of solvent (in kg)
- M (molarity): volume of solution (in L)
- x (mole fraction): total moles of all components (solution)
Mistake 5: Not explaining why volume changes with temperature
The Error: Students say "Molarity depends on temperature because volume changes" but do not explain why volume changes.
Why it happens: They give the answer without the reasoning, losing marks in "explain" type questions.
How to avoid: …
Showing the 12 most recent of 28 on this concept.
- AP EAPCET 2026Set eng-2026-05-12-AN1 markMCQQ.Chlorophyll contains 2.4% of magnesium. The number of magnesium atoms present in 2.0 g of chlorophyll is (N=6×1023 mol−1, At.wt of Mg = 24 u) (A) 1.8×1021 (B) 2.4×1021 (C) 1.2×1021 (D) 3.6×1021
›Reveal solutionSolution
A straightforward percentage-composition → moles → atoms calculation; the answer is 1.2×1021 Mg atoms.
Concept and Intuition
Percentage composition tells us the mass fraction of an element in a compound. Once we know the mass of that element, dividing by its atomic mass gives moles, and multiplying by Avogadro's number gives the actual atom count — the standard mass→mole→number chain.
Step-by-Step Solution
- Mass of Mg in 2.0 g chlorophyll =2.4%×2.0 g=0.024×2.0=0.048 g.
- Moles of Mg =240.048=0.002 mol. …
- AP EAPCET 2026Set eng-2026-05-12-AN1 markMCQQ.The molar mass of mustard gas is 159 gmol−1. The percentage by mass of sulphur and chlorine in it are respectively (Atomic weight: Cl=35.5 u, S=32 u) (A) 20.12, 44.65 (B) 44.65, 20.12 (C) 40, 24.65 (D) 24.65, 40
›Reveal solutionSolution
Mustard gas, (ClCH2CH2)2S (C4H8Cl2S, M=159), is 20.1% S and 44.65% Cl by mass.
Concept and Intuition
Percentage composition just needs the correct molecular formula and the atomic masses of the elements asked about, divided by the total molar mass.
Step-by-Step Solution
- Mustard gas is bis(2-chloroethyl) sulfide, (ClCH2CH2)2S, i.e. C4H8Cl2S.
- Molar mass check: C4=48, H8=8, Cl2=2×35.5=71, S=32. Sum =48+8+71+32=159gmol−1 — matches the given value, confirming the formula.
- Mass fraction of S =15932=0.2013⇒20.12% (rounded as in the option).
- Mass fraction of Cl =15971=0.4465⇒44.65%.
- So %S, %Cl (respectively) =20.12, 44.65. …
- AP EAPCET 2026Set eng-2026-05-14-AN1 markMCQQ.A gas mixture contains 64% methane and 36% ethane by mass. The density of the mixture at 27°C and 750 mm pressure (in g L−1) is (at. wt C=12 u,H=1 u,R=0.082 L atm K−1mol−1) (A) 0.57 (B) 0.87 (C) 0.67 (D) 0.77
›Reveal solutionSolution
Compute the mixture's average molar mass from the mass percentages, then apply the ideal-gas density formula d=PM/RT to get ≈0.77 g L−1.
Concept and Intuition
For a gas mixture, the effective (average) molar mass is the total mass divided by total moles of all species combined. Once you have that average M, the density of the mixture behaves just like a single ideal gas of that molar mass: d=PM/RT.
Step-by-Step Solution
- Assume 100 g of mixture: mass of CH4=64 g, mass of C2H6=36 g.
- Moles of CH4 (M=16 g/mol) =64/16=4 mol.
- Moles of C2H6 (M=30 g/mol) =36/30=1.2 mol.
- Total moles =4+1.2=5.2 mol; total mass =100 g.
- Average molar mass Mavg=100/5.2=19.23 g/mol.
- Convert pressure: P=750/760=0.9868 atm; T=27°C=300 K. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.At T(K), a gaseous mixture contains H2 and O2. The total pressure of the mixture is 2 bar. The partial pressure of H2 is 1.778 bar. What is the weight (w/w) percentage of H2 in the mixture ? (A) 66.67 (B) 33.33 (C) 80.00 (D) 20.00
›Reveal solutionSolution
Tests converting partial-pressure (mole fraction) data into a weight percentage using molar masses; the answer is 33.33%.
Concept and Intuition
Partial pressure is proportional to mole fraction (Dalton's law: pi=xiPtotal at constant T, V). Once we know the mole fractions of each gas, we can find the mass of each component (for an assumed total of 1 mole of gas mixture) using their molar masses, and hence the weight percentage.
Step-by-Step Solution
- Mole fraction of H2: xH2=PtotalpH2=21.778=0.889.
- Mole fraction of O2: xO2=1−0.889=0.111.
- Assume 1 total mole of gas mixture. Mass of H2=0.889×2 g/mol=1.778 g. Mass of O2=0.111×32 g/mol=3.552 g.
- Total mass = 1.778+3.552=5.33 g. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.1.84 g of a mixture of CaCO3 and MgCO3 is strongly heated to get a residue of 0.96 g. The percentage of CaCO3 in the mixture is (A) 50.34 (B) 49.66 (C) 54.34 (D) 45.66
›Reveal solutionSolution
Tests mass-balance on a two-component thermal decomposition mixture; solving simultaneous equations gives 54.34% CaCO3.
Concept and Intuition
Both carbonates decompose on strong heating: CaCO3→CaO+CO2 and MgCO3→MgO+CO2, each losing CO2 mass. Since the two salts lose mass in different fixed ratios (dictated by their molar masses), the residue mass depends on the mixture's composition — letting us solve for the unknown split.
Step-by-Step Solution
- Let mass of CaCO3=x g, so mass of MgCO3=(1.84−x) g.
- CaCO3 (M = 100 g/mol) → CaO (M = 56 g/mol): mass of CaO produced =10056x=0.56x.
- MgCO3 (M = 84 g/mol) → MgO (M = 40 g/mol): mass of MgO produced =8440(1.84−x).
- Total residue: 0.56x+8440(1.84−x)=0.96.
- Compute 8440=0.4762: 0.56x+0.4762(1.84)−0.4762x=0.96 …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.The mole fractions of glucose and water in aqueous glucose solution are 0.0244 and 0.9756 respectively. What is the weight percentage (w/w) of glucose in this solution ? (A) 40 (B) 25 (C) 20 (D) 10
›Reveal solutionSolution
Converting mole fraction to weight percentage for a glucose–water solution gives approximately 20% w/w glucose.
Concept and Intuition
Mole fraction tells us the mole ratio of components; to get weight percentage we must convert moles to mass using the molar masses (glucose M=180 g/mol, water M=18 g/mol), then take the mass fraction.
Step-by-Step Solution
- Assume a total of 1 mole of solution (mole fractions given directly as fractions of 1 mole total).
- Moles of glucose =0.0244, moles of water =0.9756.
- Mass of glucose =0.0244×180=4.392 g.
- Mass of water =0.9756×18=17.5608 g.
- Total mass of solution =4.392+17.5608=21.9528 g. …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.At T(K), a gaseous mixture contains H2 and O2. The total pressure of the mixture is 2 bar. The weight percentage (w/w) of H2 is 33.33%. What is the approximate ratio of partial pressure of H2 and O2? (A) 8 : 1 (B) 4 : 1 (C) 2 : 1 (D) 3 : 1
›Reveal solutionSolution
Converting the given weight percentage to moles (using molar masses 2 and 32 g/mol) gives a partial pressure ratio of 8:1.
Concept and Intuition
By Dalton's law of partial pressures, for a gas mixture at a common temperature and volume, the partial pressure of each component is proportional to its mole fraction (equivalently, to its number of moles). So converting mass percentages to moles (using each gas's molar mass) directly gives the pressure ratio.
Step-by-Step Solution
- Take a convenient total mass, say 3 g, so that 33.33%=1/3 works out to whole numbers: mass of H2 =1 g, mass of O2 =2 g.
- Moles of H2: nH2=21=0.5 mol
- Moles of O2: nO2=322=0.0625 mol …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.What are the mole fractions of glucose and water respectively, in 20% (w/w) aqueous glucose solution ? (C = 12 u; O = 16 u; H = 1 u) (A) 0.0244, 0.9756 (B) 0.04, 0.96 (C) 0.0636, 0.9364 (D) 0.0124, 0.9876
›Reveal solutionSolution
Taking a 100 g basis of the 20% w/w solution gives 20 g glucose and 80 g water; converting to moles and dividing gives mole fractions 0.0244 (glucose) and 0.9756 (water).
Concept and Intuition
"20% w/w" means 20 g of solute per 100 g of solution, so the remaining 80 g is solvent. Mole fraction of each component is its moles divided by the total moles of all components.
Step-by-Step Solution
- Basis: 100 g solution → 20 g glucose (C6H12O6, M=180 g/mol), 80 g water (M=18 g/mol).
- Moles of glucose =20/180=0.1111 mol.
- Moles of water =80/18=4.4444 mol.
- Total moles =0.1111+4.4444=4.5556 mol.
- Mole fraction of glucose =0.1111/4.5556=0.0244. …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.In aqueous glucose solution, the mole fraction of water is 40 times to mole fraction of glucose. What is the weight percentage (w/w) of glucose in the solution? (A) 40 (B) 30 (C) 20 (D) 10
›Reveal solutionSolution
This tests converting mole-fraction ratio into moles, then into a weight percentage. The answer is 20%.
Concept and Intuition
Mole fraction compares moles of a component to total moles, while weight percentage compares mass of a component to total mass. To go from one to the other we must first fix actual mole amounts (using the given ratio), then convert moles to grams using molar masses (glucose M=180 gmol−1, water M=18 gmol−1).
Step-by-Step Solution
- Let mole fraction of glucose =x2 and of water =x1. Given x1=40x2.
- Since x1+x2=1: 40x2+x2=1⇒x2=411, x1=4140.
- Take n2=1 mol glucose ⇒n1=40 mol water (same ratio as mole fractions since total moles cancel).
- Mass of glucose =1×180=180 g. Mass of water =40×18=720 g. …
- AP EAPCET 2025Set eng-2025-05-26-AN1 markMCQQ.The mass of a mixture containing NaCl and NaBr is 4.0 g. If Na is 30% of the total mixture, the composition of NaCl in the mixture is (Na = 23 u, Cl = 35.5 u, Br = 80 u) (A) 48% (B) 55% (C) 45% (D) 52%
›Reveal solutionSolution
This is a mixture stoichiometry problem: using the given %Na by mass to set up and solve a linear equation for the NaCl fraction, giving 45%.
Concept and Intuition
Each mole of NaCl and each mole of NaBr contributes exactly one mole of Na. So if we know the total mass of Na present, and express the unknown masses of NaCl and NaBr in terms of one variable, we can solve for the composition directly using molar masses.
Step-by-Step Solution
- Total mixture mass = 4.0 g. Na is 30% of this by mass: mass of Na =0.30×4.0=1.2 g.
- Moles of Na =231.2=0.052174 mol.
- Let mass of NaCl =x g, so mass of NaBr =(4.0−x) g.
- Molar mass of NaCl =23+35.5=58.5; molar mass of NaBr =23+80=103.
- Moles of Na from both salts: 58.5x+1034.0−x=0.052174.
- Multiply through by 58.5×103=6025.5: 103x+58.5(4.0−x)=314.35. …
- AP EAPCET 2024Set ap-2024-05-16-AN1 markMCQQ.At 293 K the density of an aqueous solution containing 120 g of urea (NH2CONH2) per dm3 is 1.02 kg dm−3. The mole fraction of urea is approximately (A) 0.038 (B) 0.962 (C) 0.02 (D) 0.98
›Reveal solutionSolution
Using the solution's density to get the total mass per litre, subtracting the urea mass gives the water mass; converting both to moles gives a urea mole fraction of about 0.038.
Concept and Intuition
Mole fraction requires the moles of each component in a defined amount of solution. Density lets us convert the given volume (1 dm³ = 1 L) into total solution mass, from which subtracting the known solute mass gives the solvent (water) mass.
Step-by-Step Solution
- Total solution mass in 1 dm³ = density × volume = 1.02 kg/dm3×1 dm3=1020 g.
- Mass of urea = 120 g (given), so mass of water = 1020−120=900 g.
- Molar mass of urea, NH2CONH2: 2(14)+4(1)+12+16=28+4+12+16=60 g/mol.
- Moles of urea =120/60=2 mol.
- Moles of water =900/18=50 mol. …
- AP EAPCET 2024Set eng-2024-05-18-FN1 markMCQQ.The concentration of 1L of CaCO3 solution is 1000 ppm. What is its concentration in mol L−1? (Ca = 40 u, O = 16 u, C = 12 u) (A) 10−3 (B) 10−1 (C) 10−4 (D) 10−2
›Reveal solutionSolution
Converting 1000 ppm of CaCO3 (≈ 1 g/L for a dilute aqueous solution) into molarity using M = 100 g/mol gives 10−2 mol/L — option (D).
Concept and Intuition
"ppm" (parts per million) for a dilute aqueous solution is conventionally taken as mg of solute per litre of solution (since the solution's density is essentially that of water, 1 g/mL, at these very low concentrations). Once we have the mass concentration in g/L, dividing by the molar mass converts it to molarity directly.
Step-by-Step Solution
- 1000 ppm = 1000 mg solute per L of solution = 1 g/L (for a dilute aqueous solution where density ≈ 1 g/mL).
- Molar mass of CaCO3: 40(Ca)+12(C)+3×16(O)=40+12+48=100 g/mol. …
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