Question

(A) how would you prepare 500 mL of 0.1 M NaH2Po4 starting with solid salt?

(B) when you look for NaH2PO4 in your lab, you find a jar with (NaH2PO4)(2H20). Can you use this chemical instead, and if so how much do you need to weigh out?

Answer 1

(A) Weigh 6.0 g of NaH2PO4 and dissolve in water and make the volume to 500 mL with water

Molar mass of NaH2PO4 = 119.98 g/mol

Concentration of NaH2PO4 required = 0.1 M

Volume of NaH2PO4 solution required = 500 mL

Mass of NaH2PO4 required = 119.98 g/mol x 0.1 M x (500/1000 mL) = 5.999 g = 6.0 g

(B) Yes we can use becasue it is a hydrated salt of NaH2PO4, weigh 7.8 g of NaH2PO4.2H2O and dissolve in water and make the volume to 500 mL with water

Molar mass of NaH2PO4.2H2O = 156.01 g/mol

Concentration of NaH2PO4 required = 0.1 M

Volume of NaH2PO4 solution required = 500 mL

Mass of NaH2PO4 required = 156.01 g/mol x 0.1 M x (500/1000 mL) = 7.8005 g = 7.8 g