Question

A 35.0 mL sample of HCl solution is placed in a flask, with a few drops of phenolphthalein indicator. Of 45.9 mL of a 0.200 M solution is required to reach the endpoint, what was the concentration of the HCl solution

Answer 1

we know that

at endpoint

moles of acid = moles of base

also

moles = molarity x volume

so

at end point

Ma x Va = Mb x Vb

given

Va = 35

Mb = 0.2

Vb = 45.9

so

Ma x 35 = 0.2 x 45.9

Ma = 0.2623

here acid is HCl

so

the concentration of the HCl solution is0.2623 M