Question

In a titration, you reach the equivalence point of pH = 7.0 after adding 35 mL of KOH (0.05 M) to a 60 mL nitric acid solution of unknown concentration. What is the concentration of the HNO3?

a) 0.025 M
b) 0.030 M
c) 0.035 M
d) 0.040 M

Answer 1

The concentration of HNO3 is calculated using the stoichiometry of the neutralization reaction with KOH. By determining the moles of KOH at the equivalence point, the concentration of HNO3 in the original solution is found to be approximately 0.030 M.

Step-by-step explanation:

The concentration of HNO3 in the original solution can be calculated using the stoichiometry of the neutralization reaction between KOH and HNO3. Since the reaction is KOH + HNO3 → KNO3 + H2O, the molar ratio of KOH to HNO3 is 1:1. To find the concentration of HNO3, the moles of KOH used at the equivalence point are determined and then used to calculate the moles of HNO3 that must have been present in the original solution.

The moles of KOH are calculated by multiplying its volume in liters by its molarity (0.035 L × 0.05 M = 0.00175 moles). Since the molar ratio is 1:1, there were also 0.00175 moles of HNO3 in the 60 mL solution. To find the concentration of HNO3, divide the moles of HNO3 by the volume of solution in liters (0.00175 moles ÷ 0.060 L = 0.0292 M or approximately 0.030 M, which corresponds to option b).