Question

how many grams of sodium phosphate would you need to make 275 ml of 7.75 m stock solution?

Answer 1

Sodium Phosphate (NaH2PO4) Molecular weight is = 23+2+31+64= 120

molality= moles of solute/ kg of solvent

Moles of solute (NaH2PO4)= m/120 , m= mass of NaH2PO4.

volume of water= 275ml, assuming the density of water to be 1 g/cc , mass of water =275 gm =0.275kg

molality= 7.75 moles/ kg solvent

1 kg solvent contains 7.75 moles of NaH2PO4

0.275 kg solvent contains 7.75*0.275=2.13 moles of NaH2PO4

Moles = mass/Molecular weight ( if it is not monohydrate)=

2.13= mass/120

Mass =2.13*120=255.6 gms

If it is NaH2PO4.H2O, molecular weight = 138

Mass =2.13*138=293.94 gms