Question

1.) A biochemistry student wants to make 4 liters of a phosphate solution using 48.6 g Na2HPO4 (FW=141.96) What is the concentration of this solution?

2. When she reaches the storeroom she finds that there is no Na2HPO4, however there is a bottle of hydrate Na2HPO4 * 7H20. (FW=268.07) how many grams of this salt should she use in order to make 4 liters with the same phosphate concentration as above?

Answer 1

1)
Given:
V = 4[L]
m1 = 48.6[g]
w1 = 141.96[g/mol]
Use:
c = m1/(w1*V)

2)
Given:
w2 = 268.07
Use:
m2/(w2*V) = m1/(w1*V)
.: m2 = m1*w2/w1

Answer 2

I'm actually still confused by this