Question

A buffer solution of pH 7.30 will be prepared from a 50.00ml solution of 0.075 M NaH₂PO4 (K₂= 6.3 x 108). How many grams of NaH₂PO4 must be dissolved in the solution to get the desired pH? (molar mass of NaH₂PO4= 119.0g/mol).

Answer 1

One of the material of making the buffer is the acid $NaH_2PO_4$

The number of moles in $50ml \>$ of the acid:

$=\frac{50}{1000}\times0.075=0.00375\>moles$

This can be obtained by dissolving $m$

grams in water and topping up to $50ml$

$\frac{m}{119}=0.00375$

$m=119\times0.00375$

$=0.44625g\quad(in\>50ml)$