[antoinetta]

Hi, I'm just having trouble starting this question:

A 248.8g sample of ground water is analyzed for calcium.  Calcium ions in the sample is first precipitated and filtered off as NH4CaPO4-7H20.  This precipitate is dried and heated, releasing water and ammonia to yield anhydrous calcium pyrophosphate Ca2P2O7.  The mass of Ca2P2O7 obtained is 0.0459g.  What is the calcium content of the ground water in parts per million?

[enahs]

Take mass of product and convert to moles. Use stoichiometry to determine the number of moles in the original solution. Then convert to ppm.

[antoinetta]

So taking your advice, I found that the moles of Ca2P2O7 is 1.8059*10^-4 mols

I'm just having trouble with the equation:  is this what I am supposed to balance?

NH4CaPO4-7H29 ->Ca2P2O7 + H2O + NH3

[Borek]

I'm just having trouble with the equation:  is this what I am supposed to balance?

NH4CaPO4-7H29 ->Ca2P2O7 + H2O + NH3

There is no need for balancing. You are interested in calcium mass only, you can find it just knowing mass of Ca₂P₂O₇. enahs proposition (while leads to the correct answer) is unnecesarilly long.

enahs: you don't have to do all stoichiometric calculations using molar ratios, you can use molar mass ratios as well. See stoichiometric calculations using ratios.

[enahs]

Yeah, I was not thinking when I posted. I made it harder then it actually is. My bad.

[enantiomorph]

Hi there, I think I can get this one for antoinetta.  By following enahs approach:

The balanced chemical equation would be, 2NH4CaPO4.7H2O -> 15H2O + 2NH3 + Ca2P2O7
The number of moles of Ca2P2O7 = (0.0459g)/(40.08 x 2+2 x 30.97+7 x 16)g/mol
For every one mole of the hydrate, there is one mole of Ca.
From the balanced chemical equation, the number of moles of the hydrate = twice the number of moles of Ca2P2O7
Thus, the mass of Ca = (molar mass of Ca) x (2 x number of moles of Ca2P2O7)
In one step, mass of Ca = (40.08 g/mol) x [ 2 x  (0.0459g)/(40.08 x 2+2 x 30.97+7 x 16)g/mol ]
To find the concentration in ppm, simply divide the (mass of Ca) by the given mass of 248.8 g and multiply it by 1,000,000.