1. (7.1) Stoichiometry-Part 2

2. Mass Calculations
We already know that in balanced chemical equations, coefficients represent particles and moles
But, if we know molar masses, we can also determine mass relationships

3. Step by Step Solving Stoichiometric Problems
Write out a balanced equation.
Convert given mass (or # of particles) to moles.
Calculate the moles of required substance based on the moles of given substance.
Convert the moles of required substance to mass (or # of particles) based on the question.

4. General Plan for Solving Stoichiometric Calculations
mass of first substance
mass of second substance
Molar
mass
n = m M
Molar
mass
m = n x M
Mole ratio
moles of first substance
moles of second substance

5. Sample Problem
Phosphorous pentachloride, PCl5, reacts with water to form phosphoric acid, H3PO4(aq), and hydrochloric acid, HCl(aq)
PCl5(s) + 4H2O(l)  H3PO4(aq) + 5HCl(aq)
a) What mass of PCl5 is needed to react with an excess quantity of H2O to produce 23.5g of HCl?
(Ans: 26.9 g)
b) How many molecules of H2O are needed to react with 3.87g of PCl5?
(Ans: 4.48 x 1022 molecules)

6. Page 244 # 11-14
Page 246 # 15-18
Page 248 # 19-22
(see answers Page 273)
Page 244 (#11-13)
Page 246 (#15-18)
Page 248 (#19-21)