A student determines an average concentration of 0.04565 for their diluted sulfuric acid sample. If the diluted acid sample was prepared by diluting a 20.00 mL sample of stock acid to a total volume of 100.00 mL, what is the concentration of the stock acid?

0.22825 M or 0.22825 mol/dm3

Explanation:

From the principle of Dilution, the number of moles of solute remains the same in spite of the increase in volume of solution.

The number of moles of H2SO4 before dilution is given by

nb = CbVb/1000 ... (1)

where

nb = number of moles of acid before dilution

Cb = concentration of acid before dilution, Cb = ?

Vb = volume of acid before dilution, Vb = 20mL

After dilution, we have:

na = CaVa/1000 ... (2)

Where:

na = number of moles of acid after dilution

Ca = concentration of acid after dilution, Ca = 0.04565 M

Va = volume of acid after dilution, Va = 100mL

According to the principle of dilution:

nb = na ... (3)

CbVb/1000 = CaVa/1000

CbVb=CaVa ... (4)

making Cb the subject of the formula, we have:

Cb = CaVa/Vb

= 0.04565 x 100/20

= 0.22825 mol/dm3 or 0.22825 M