A student is performing a titration to determine the concentration of a 20.0 mL sample of hydrochloric acid. What did the student determine the concentration of the HCl to be if the end point was reached after 50.0 mL of 0.1 M NaOH was added.

0.25M HCl

Explanation:

The reaction of HCl with NaOH is:

HCl + NaOH ⇄ H₂O + NaCl

Where 1 mole of HCl reacts per mole of NaOH

The end point was reached when the student added:

0.0500L × (0.1mol / L) = 0.00500 moles of NaOH

As 1 mole of HCl reacted per mole of NaOH, moles of HCl present are:

0.00500 moles HCl

The volume of the sample of hydrochloric acid was 20.0mL = 0.0200L, and concentration of the sample is:

0.00500 mol HCl / 0.0200L = 0.25M HCl