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Chemistry

Calculate the percentage of phosphorus in

(a) Calcium hydrogen phosphate Ca(H2PO4)2

(b) Calcium phosphate Ca3(PO4)2

Mole Concept

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Answer

(a) Molecular mass of Ca(H2PO4)2

= Ca + 2[2H + P + 4O]

= 40 + 2[2(1) + 31 + 4(16)]

= 40 + 2[2 + 31 + 64]

= 40 + 194

= 234

234 g of Ca(H2PO4)2 contains 62 g of P

∴ 100 g of Ca(H2PO4)2 contains

= 62234\dfrac{62}{234} x 100 = 26.5%

(b) Molecular mass of Ca3(PO4)2

= 3Ca + 2[P + 4O]

= (3 x 40) + 2[31 + 4(16)]

= 120 + 2[31 + 64]

= 120 + 190

= 310

310 g of Ca3(PO4)2 contains 62 g of P

∴ 100 g of Ca3(PO4)2 contains

= 62310\dfrac{62}{310} x 100 = 20 %

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