Question

Gravimetric Determination: Why is my percent of P₂O₅ much higher than the guaranteed minimum percent? (What did I do wrong?)

Weight of Sample:
Trial 1 (1.9670g)
Trial 2 (1.9318g)

Weight of filter paper and MgNH₄PO₄x6H₂O
Trial 1 (4.4299g)
Trial 2 (4.4190g)

Weight of filter paper
Trial 1 (2.4629g)
Trial 2 (2.4872g)

Weigh of MgNH₄PO₄x6H₂O
Trial 1 (1.9670g)
Trial 2 (1.9318g)

Average P in Sample
12.63%

Average P₂O₅
28.92%

The label said 24-8-16

Answer 1

In gravimetric methods, if sample is not dried completely, weight of compound could differ from actual mass. Also,

P₂O₅ (diphosphorous pentoxide) is a deliquescent compound, that means it can absorb moisture from air /atmosphere that may cause increase in weight of the sample.

or, this could be due to lack of proper washing with suitable solvent, due to which other compounds present in mixture is precipitating along with your compound i.e. co-precipitation may occur.

Post-precipitation chances are also there, which may lead to increase in weight pf sample.