According to Reference Table V, which of these acids has the smallest ionization constant (Ka )? A) HCl B) HBr C) HNO2 D) HNO3?

The answer: C) HNO₂

You may refer to the Ka values in the table or according to the deduction below.

The larger the value of Ka, the stronger the acid is. HCl, HBr and HNO₃ are strong acids and thus their Ka values are large, while HNO₂ is a weak acid and thus its Ka value is small. Among the four acid, HNO₂ has the smallest Ka value.