Q.Compare the acidic character of ethyne, ethene and ethane, giving the reason based on percentage $s$-character.

Ethyne's acidic hydrogen sits on an $sp$-hybridised carbon ($50\%$ $s$-character), ethene's on an $sp^2$-hybridised carbon ($33\%$ $s$-character), and ethane's on an $sp^3$-hybridised carbon ($25\%$ $s$-character). Since an $s$ orbital's electron density is held, on average, closer to the nucleus than a $p$ orbital's, a hybrid orbital with more $s$-character behaves as though the carbon holding it is more electronegative, so the lone pair left on the carbanion after deprotonation is held more tightly and stabilised more effectively the higher the $s$-character of the orbital it occupies. A more stabilised conjugate base corresponds directly to a stronger acid, so acidity runs in the same order as $s$-character: ethyne ($sp$, $\text{p}K_a \approx 25$) is the most acidic, followed by ethene ($sp^2$ …

List the hybridisation and % s-character at the relevant carbon in each compound, and link higher s-character directly to greater stabilisation of th …

Reversing the trend (assuming p-character, being 'less tightly held', stabilises a negative charge better -- it is the opposite: s-character, being clos …