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Question

A flask contains 10.0 mL of HCL and a few drops of phenolphthalein indicator. The buret contains 0.250 M NaOH. It requires 30.3 mL of the NaOhH solution to reach the end point of the titration. What is the initial concentration of HCL.

Answer 1

Write a balanced equation for the reaction:

HCl + NaOH ==> NaCl + H2O ... balanced equation

moles NaOH used = 30.3 ml x 1 L / 1000 ml x 0.250 mol / L = 7.58x10^-3 mols NaOH

Since the mole ratio in the balanced equation for HCl : NaOH is 1 : 1,

that means there are 7.58x10^-3 mols HCl present

The concentration of HCl is given as moles HCl / L

the moles are 7.58x10^-3 mols

the liters are 10.0 ml HCl x 1 L / 1000 ml = 0.010 L

Concentration HCl = 7.58x10^-3 mols / 0.010 L = 0.758 M