Chem Class- November 12, 2009

We:

• Went over the answers for the 10-2 Practice Problems
• Did some more review on what we learned so far in the unit (Exams are coming up)
• Learned the percent mass of elements in compounds

Now we can find the % by the mass of each element in a compound. We can also find the mass of an element in a given mass of a compound.

Here's a Video on calculating the Percent Compostition. At about 4:24, however, they start empirical formulas (which we didn't learn yet!)



Here's some sample questions we made up for practice:

1) Find the percent of Iron by mass in FeCl2:
Fe = 19/ Cl = 2(35.5)/ = 19 + 71 = 90 g/mol
% Iron = (19/90) x 100 = 21.1 %

2) Find the mass of Sodium contained in 15.0 g sample of NaOH:
Na = 23/ O = 16/ H = 1/ = 23 + 16 + 1 = 40g/mol
% of Sodium = (23/40) x 100 = 57.5%
15.0 g x 0.575 = 8.6 g

3) Calculate the percentage composition of (NH4)3PO4:
Molar mass =149.0 g/mol (N = 42, H = 12, P = 31, O = 64)
% N = (42.0 g/149.0 g) x 100 = 28.2 %
% H = (12.0g/149.0 g) x 100 = 8.1 %
% P = (31.0 g/149.0g) x 100 = 20.8 %
% O = (64.0 g/ 149g) x 100 % = 43.0 %