question about hydrates?

Question

a hydrate of sodium phosphate contains 49.7% water by weight.

a) how many grams of water and how many grams of anhydrous Na3PO4 are in 1000. grams of this sample?

b) In this same 1000. gram sample, how many moles of water and how many moles of anhydrous Na3PO4 are present.

c. What is the formula of the hydrate?

(i need the computation for fully understanding) thanks

Answer 1

Let sodium phosphate hydrate be called SPH.

Atomic weights: Na=23 P=31 O=16 H=1 Na3PO4=164 H2O=18

a) 100%SPH - 49.7%H2O = 50.3%Na3PO4
497g H2O and 503gNa3PO4

b) 497gH2O x 1molH2O/18gH2O = 27.6 moles H2O
503gNa3PO4 x 1molNa3PO4/164gNa3PO4 = 3.06 moles Na3PO4

c) 27.6molH2O/3.06molNa3PO4 = 9.2 moles H2O per mole Na3PO4.

Within experimental error, the formula is Na3PO4*9H2O

Answer 2

a) the weight of water is 1000*49.7/100 =497 g
the weight of the phosphate is 1000 -497 =503 g

b) a mole of water has a molecular mass of 18 . so in 497 g, you have 497/18 = 27.61 moles
the molecular weight of Na3PO4 = 3*23+31+4*16 =164g .
497/164 =3.03 moles

c)As water represents 49.7% and phosphate 50.3, the molar mass of water in the hydrate is 164*49.7/50.3= 162g and 162 /18 =9
formula Na3PO4, 9 H2O