Q.Arrange the ligands $\text{I}^-$, $\text{H}_2\text{O}$, $\text{NH}_3$, and $\text{CN}^-$ in increasing order of ligand field strength according to the spectrochemical series.

[!TLDR] Read the ligand off the spectrochemical series, left (weak) to right (strong). [!ANSWER] $\text{I}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{CN}^-$ (increasing field strength).

The spectrochemical series (Section 5.3) is an experimentally determined ranking of ligands by how strongly they split a metal's d-orbital energies. Locating each of the four given ligands on the series: $\text{I}^-$ sits at the extreme weak-field end (large, highly polarizable halide ion, weak splitting); $\text{H}_2\text{O}$ sits in the intermediate region (a moderate-field neutral O-donor); $\text{NH}_3$ sits further along, in the stronger-field region (moderate-to-strong N-donor); and $\text{CN}^-$ sits at the extreme strong-field end (one of the very strongest-splitting ligands known, alongside $\text{CO}$). Reading them off in the order the series places them gives the increasing-field-strength sequence directly. [!ANSWER] Increasing ligand field strength: $\text{I}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{CN}^-$.

Locate each ligand's position on the memorized spectrochemical series and read off the requested order directly.

Confusing field strength with the ligand's charge or its position in a reactivity/nucleophilicity series — field strength is its own separately-measured, empirical property and does not track simply with charge (e.g. neutral $\text{CO}$ is a stronger-field ligand than the negatively charged $\text{I}^-$).