WEBVTT

1
00:00:00.000 --> 00:00:07.651
Here is the problem: Balance the chemical equation Al plus HCl → AlCl3 plus H2.

2
00:00:08.101 --> 00:00:12.803
Balance atoms of every element, then use the coefficients as mole ratios.

3
00:00:13.603 --> 00:00:22.903
Write an atom balance for each element. Give each species an unknown coefficient; atoms of every element must be equal on both sides.

4
00:00:23.353 --> 00:00:33.349
Solve the linear system with the smallest whole numbers. Exact elimination leaves one free coefficient; scaling by the least common denominator gives integers.

5
00:00:33.799 --> 00:00:34.948
Balanced equation.

6
00:00:35.748 --> 00:00:42.911
So the answer is 2Al plus 6HCl → 2AlCl3 plus 3H2.

7
00:00:43.361 --> 00:01:00.439
Coefficients in a balanced equation count particles (moles), not grams. Convert grams to moles, apply the mole ratio, then convert back to grams with the other species prime molar mass. Assumes the reaction goes to completion with the known species fully consumed (or formed).

8
00:01:01.239 --> 00:01:09.006
Al: 2 equals 2, H: 6 equals 6, Cl: 6 equals 6, which checks out.