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Hard Reacting Masses!?

I understand how to do simple mole ratio question e.g 1:2 2:1 1:4 etc...
But how do i work out hard questions?The bit i find hard is the ratio bit when the ratios are like 13:8 etc... i will give a sample question.
calculate the mass of phosphoric acid required to make 30g of hydrogen.
6Li + 2H3PO4 ---- 2Li3PO4 +3H2. How would you find the moles of the acid with the hard ratio of 3:2?

make it a 1:x ratio

Original post by Adham5332

If you truly understand how to solve simple mole ratio questions, then this question that you posted should not be an issue for you.

1) Find the number of moles of hydrogen gas by using the formula: mole = mass/Mr
2) Compare the mole ratio of hydrogen gas: phosphoric acid and that ratio is 3:2, based on your balanced equation.
3) Number of moles of phosphoric acid = (no. of moles of hydrogen gas / 3 ) x 2
4) To find the mass of phosphoric acid, use the formula: mole = mass /Mr. You already know the number of moles of phosphoric acid based on step 3 and you also know the molecular formula of phosphoric acid, which would allow you to work out its Mr. Hence, mass can be determined.

Hope it helps. Cheers.

Reply 3

spingu101

remember: "divide by where you are (what you have to start) and times by where you need to be (yield to be calculated). Khanacademy has some really good videos on this