Consider the given acid ionization constants. Identify the strongest conjugate base. Acid KaKa HNO2(aq)HNO2(aq) 4.6×10−44.6×10−4 HCHO2(aq)HCHO2(aq) 1.8×10−41.8×10−4 HClO(aq)HClO(aq) 2.9×10−82.9×10−8 HCN(aq)HCN(aq) 4.9×10−104.9×10−10

HCN(aq) 4.9×10−10

Explanation:

There is a relationship between the strength of an acid (or base) and the strength of its conjugate base (or conjugate acid):

The stronger the acid, the weaker its conjugate base. The weaker the acid, the stronger its conjugate base. The stronger the base, the weaker its conjugate acid.

So in short, we are to find the weakest acid amongst the options given. What is the relationship between acid ionization constant and its strength?

The larger the Ka, the stronger the acid and the higher the H+ concentration at equilibrium.

From this, the acid with the lowest Ka value is our acid of interest,

Upon observing the options, our answer is HCN, with a dissociation constant of 4.9×10−10

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