Chemistry Stoichiometry mass to mass calculations

Question

. If 25.00 grams of ammonium phosphate reacts with some magnesium hydroxide, what mass of

   water and ammonia will be produced? (equation is not balanced!!!!!)

                     Mg(OH)₂ + (NH₄)₃(PO₄) ®  Mg₃(PO₄)₂ + NH₃ + H₂O  ( not balanced)

Answer 1

So Jayla L.,

Do as you always (should!) do in these problems.

1) Balance the equation

2) Convert any given masses into moles units, using molar mass for each respective material.

3) If there is a limiting reactant, find it and perform all subsequent operations based on using just that much of it (here, it says "some Magnesium hydroxide" -- you could go wild and figure how much magnesium hydroxide would be needed, and stage an answer around that -- but I suspect that " excess magnesium hydroxide" is implied, so assume that the ammonium phosphate is limiting.)

4) Ratio the input moles of limiting reactant through to products, using ratios of coefficients of the materials in the balanced reaction. That gives you moles of each product.

5) Convert the products to a mass basis, using the molar mass of each material.

So recapitulate what you did here. The key take-away is: all reactions go through as moles to moles. That represents not only convenient lab-scale quantities, but also the ratios at the molecular scale! Molecules react, not masses of stuff! And moles is the handle we use on getting the right amounts of molecules.

--Cheers, --Mr. d.