1.A student was given a sample of water that contained 5.65x10-4 moles of phosph

Question

1.A student was given a sample of water that contained 5.65x10-4 moles of phosphate (HPO42-). They added an excess magnesium (Mg2+) and ammonium (NH4+) to the water sample to form struvite. Determine the expected mass of struvite the student could obtain from this sample. (SHOW THE WORK TO RECEIVE FULL MARK)

2. One of the steps in this experiment asks you to prepare a solution of MgSO4·7H2O in water. Suppose a student weighed out 10.0 g of MgSO4·7H2O, mixed it with 75 mL of H2O and the final volume of the solution was 85 mL. Calculate the percent mass by volume of the MgSO4·7H2O solution the student made. (SHOW THE WORK TO RECEIVE FULL MARK)   

Explanation / Answer

Ans. #1. Struvite = NH4MgPO4.6H2O

1 mol struvite consists of 1 mol PO43-.

In excess of NH4+ and Mg2+, all the PO43- would completely be converted into struvite.

Note that 1 mol HPO42- yields 1 mol PO43-

So,

            Moles of struvite formed = 5.65 x 10-4 mol = Moles of HPO42- taken

And,

Mass of struvite formed = Moles x Molar mass

                                    = 5.65 x 10-4 mol x (245.406542 g/ mol)

                                    = 0.1387 g

Therefore, expected mass of struvite formed = 0.1387 g

#2. A % (w/v) is defined as the mass of solute in grams per 100 mL of solution.

% [MgSO4.7H2O] (w/v) = (Mass of salt in gram / Volume of solution in mL) x 100

                                                = (10.0 g / 85.0 mL) x 100

                                                = 11.76 % (w/v)

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