Question

A certain concentrated HCl solution has a density of 1.27 g/mL and is 37.5% HCl by mass. What is the molarity of this concentrated HCl solution? Show your calculations.

As phenothalin acid is titrated, the pink color forms where the drops first enter the solution but disappears as the solution is mixed. Explain why this happens.

If a pipet used to measure an unknown acid was not clean, and some of the unknown acid remained above that left in the tip. Will the calculated molarity of the unknown acid be erroneously high, low, or not affected? Explain your answer.

Answer 1

a)

Molarity of a solution , M = ( 10 x density x percent of purity ) / molar mass

Given that density of HCl = 1.27 g/mL

The percent of purity = 37.5 %

The molar mass of HCl = At.mass of H + At.mass of Cl

                                   = 1.0 + 35.5   

                                   = 36.5 g/mol

=> Molarity = (10 x 1.27 x 37.5) / 36.5

= 13.05 M

b)

When an acid solution is titrated by adding a small amount of base. By adding a drop into an acidic solution, the conentration of base is more at that time. So, as a result of this, the indicator appears its colour lightly. After some time the basic drop spreads over the acidic solution leading to decrease in its concentration so the colour also disappears.

c)

If we are titrating an unknown acid with a standardized base, the calculated molarity of the acid will not be affected by that above the tip which was left in the pipette, because the amount or the number of mill equivalents of base will be equivalent to the acid added from the pipette regardless.

However, if the acid in the pipette is dirty because of the pipette being contaminated with another substance, then the molarity will be erroneously high.