Given :
Molarity of [tex]Na_3PO_4[/tex] , M = 0.213 M .
To Find :
How many ml of 0.213 M of [tex]Na_3PO_4[/tex] are required to deliver 66.4 mmol [tex]Na^+[/tex].
Solution :
Volume required :
[tex]V=[\text{ 66.4 mmol of }Na^+] + [\dfrac{\text{1 mm of }Na_3PO_4}{\text{3 mmol of }Na^+}]+[\dfrac{\text{1 ml }Na_3PO_4}{\text{0.213 mmol of }Na_3PO_4}][/tex]
So ,
[tex]V=\dfrac{66.4}{3\times 0.213}\ ml\\\\V=103.91\ ml[/tex]
Therefore , volume of [tex]Na_3PO_4[/tex] required is 103.91 ml .
Hence , this is the required solution .