WEBVTT

1
00:00:00.000 --> 00:00:05.155
Here is the problem: How many moles are in 36 g of H2O?

2
00:00:05.605 --> 00:00:10.516
Add the atomic weights of all 3 atoms in one formula unit of H2O.

3
00:00:11.316 --> 00:00:21.533
Count the atoms of each element. Subscripts multiply everything inside the brackets before them, and a hydrate coefficient multiplies the whole water (or other) unit.

4
00:00:21.983 --> 00:00:29.030
Look up each atomic weight (g/mol). IUPAC standard atomic weights (abridged, conventional values).

5
00:00:29.480 --> 00:00:32.510
Multiply each count by its atomic weight.

6
00:00:32.960 --> 00:00:34.400
Add the contributions.

7
00:00:35.200 --> 00:00:37.081
Mass percent of each element.

8
00:00:37.531 --> 00:00:39.609
Convert the sample mass to moles.

9
00:00:40.059 --> 00:00:41.777
Number of formula units.

10
00:00:42.577 --> 00:00:54.384
So the answer is M(H2O) equals 18.015 g/mol; 36 g equals 1.99833 mol.

11
00:00:54.834 --> 00:01:06.258
The molar mass is the mass of one mole of formula units. It equals the sum of the atomic weights of every atom in the formula, and numerically matches the formula mass in daltons (u).

12
00:01:07.058 --> 00:01:11.087
Percent composition adds to 100.00 percent.

13
00:01:11.537 --> 00:01:15.275
Elements: hydrogen times 2, oxygen times 1.