Stiochiometry

In balanced equations, the coefficients represents the number of moles. To find the number of moles of a compound or element in a balanced equation, we divide the given mole by the coefficient.

Example - : If 0.20 mol of methane reacts with oxygen, how many moles of each product are produced?

- first write a balanced equation, be sure that the equation is correctly balanced:
2 CH4 + 4O2 -> 2 CO2 + 4 H2O
- to find the number of moles of CO2: multiply the number of moles of methane by CO2's coefficient which is 2, and divide by the number of moles of methane.
(0.20 mol CH4 x 2 mol CO2) / (2 mol CH4) = 0.4 mol CO2

When converting moles to mass, one more step is needed (g/mol)

Example - : How many grams of water are produced if 1.0 mol of phosphoric acid is neutralized by barium hydroxide?

- first write a balanced equation:
2 H3PO4 + 3 Ba(OH)2 -> Ba(PO4) + 6 H2O
- to find the number of moles of H2O: multiply the number of moles of H3PO4 (phosphoric acid) by H2O's coefficient which is 2, and divide the number of moles of H3PO4
(1.0 mol H3PO4 x 6 H2O) / (2 mol H3PO4) = 3.0 mol H2O
- now multiply H2O's molar mass and divide by mol (g/mol)
(3.0 mol x 18 g) / (1 mol) = 54 g H20

Here's a video of Chemguy solving many moles of silver chloride forms when 2.6 mol of KCl reacts with excess silver nitrate in solution:


That is all. Mr. Doktor is the best!

-- Jael Lumba