q 30, P223 of conquering chemistry, need assistance. (1 Viewer)

[WouldbeDoctor]

A chem teacher (who is a bad teacher) said that he teachers yr 10 students to do this question... Anyhow I can only half do the question . I present you BOSer's without further adue.

30.)

The phosphate content of a laundry washing powder was determined as followes. 6.92 g of the pwoweder was dissolved in approximately 5000 m L water. A slight excess of a solution of magnesium chloridge in an ammonia, ammonium chloride buffer was added to this with stirring until precipitation weas cokplete. The precipitatewas filtered off and dried; its mass was found to be 1.83 g. The precipitate is pure magnesium ammonium phoshate hexahydrate of formula, Mg(NH4) PO4.6H2O. Calculate the niumber of moles of phosphorus in tjhe precipitate and hence the percentage phosphorus in the original laundry powder,.
The package this laundry powder came in carried a large letter P, meaning that its phosphorus content was less than 5% . Is this labelling justified?

 

[Riviet]

Percentage mass of phosphate in magnesium ammonium phosphate hexahydrate=(31/245.3) x 1.83

.'. n=m/M
={(31/245.3) x 1.83}/245.3
=0.007460252
~7.46x10^-3 moles [back of book's answer]

.'. mass of phosphate = n x M
=(7.46x10^-3) x 31
=0.23126g

.'. percentage of phosphate in laundry powder = (0.023126/6.92) x 100%
= 3.34% [back of book's answer]

Therefore, the labelling is justified. I hope that helps.