Question

Design a procedure to prepare 75 ml of 0.10 M potassium dihydrogen phosphate solution (KH2PO4) and 75 ml of 0.10 M dipotassium hydrogen phosphate solution (K2HPO4) from the solids. (Hint: What mass of the above solids is required to make 75 ml of a 0.10 M solution?)

Answer 1

To prepare 75 ml of 0.10 M potassium dihydrogen phosphate solution

Using formula for molarity (M),

M = moles/L of solution = grams of solute/(molar mass x L of solution)

with,

molar mass of KH2PO4 = 136.086 g/mol

L of solution = 75 ml = 0.075 L

molarity of final solution = 0.10 M

we get,

grams of KH2PO4 required = 0.10 M x 136.086 g/mol x 0.075 L = 1.02 g

So dissolve 1.02 g of KH2PO4 in 75 ml to get 0.10 M solution

To prepare 75 ml of 0.10 M dipotassium hydrogen phosphate solution

Using formula for molarity (M),

M = moles/L of solution = grams of solute/(molar mass x L of solution)

with,

molar mass of K2HPO4 = 174.2 g/mol

L of solution = 75 ml = 0.075 L

molarity of final solution = 0.10 M

we get,

grams of K2HPO4 required = 0.10 M x 174.2 g/mol x 0.075 L = 1.31 g

So dissolve 1.31 g of K2HPO4 in 75 ml to get 0.10 M solution