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Balancing Chemical Equations: Simple Example

I have a lot of people asking for help with balancing chemical equations. Below is my personal method, with a simple example. Click here for a PDF of a redox example.

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Directions:

  1. Make a table that shows how many of each element there are on each side of the equation.
  2. Identify an atom that is both out of balance and located in only one molecule on the left, and only one molecule on the right. (If no such atom exists, try to find one that is only in one molecule on one side, even if it is in more than one on the other side.) Start by adding coefficients that balance this atom on both sides. Cross off and update the numbers in your table to reflect the new totals for each atom.
  3. If that was not enough to balance the equation, proceed to the next atom that is in the fewest number of molecules, and repeat Step 2. Continue to do this until all atoms are balanced.
  4. Double-check by re-adding the totals for each atom to ensure that your answer is correct.

Example:

___KI(aq) + ___Pb(NO3)2(aq) ___ PbI2(ppt) + ___ KNO3(aq)

Step 1:

1

K

1

1

I

2

1

Pb

1

2

NO3*

1

*NO3 (nitrate) can be listed as one unit here because it does not separate. If nitrogen or oxygen appeared separated in the product, or if nitrate was present in the product in addition to oxygen or nitrogen appearing in some other part of this product, then this would not work. NO3 is the same on both sides, so we are able to treat it like a single unit for the sake of balancing this equation.

Step 2:

Iodine and nitrate are the only things out of balance here. Iodine is only in one molecule on the left and only in one molecule on the right. The same is true of nitrate. This means it doesn’t matter which one we start with. Let’s try starting with iodine, chosen arbitrarily:

_2_KI(aq) + ___Pb(NO3)2(aq) ___ PbI2(ppt) + ___ KNO3(aq)

2    1

K

1

2    1

I

2

1

Pb

1

2

NO3*

1

At first glance, this might seem wrong because the potassium (K) is no longer balanced. Take a look at what else is not balanced: nitrate. Nitrate and potassium happen to be in the same molecule on the right, so the next step is to choose a coefficient for that molecule that balances both potassium and nitrate if possible. Luckily, it is!

Step 3:

_2_KI(aq) + ___Pb(NO3)2(aq) ___ PbI2(ppt) + _2_ KNO3(aq)

2    1

K

1    2

2    1

I

2

1

Pb

1

2

NO3*

1    2

This looks balanced now, according to our accounting table. The last step is to double-check to make sure it is right.

Step 4: To check your work, translate the formula into an equation for each element or molecule.

_2_KI(aq) + ___Pb(NO3)2(aq) ___ PbI2(ppt) + _2_ KNO3(aq)

Potassium:

(2 X 1) + 0 0 + (2 X 1)
2
2
Therefore, potassium is correct.

Iodine:

(2 X 1) + 0 (1 X 2) + 0
2
2
Therefore, potassium is correct.

Lead:

0 + (1 X 1) (1 X 1) + 0
1
1
Therefore, potassium is correct.

Nitrate:

0 + (1 X 2) 0 + (2 X 1)
2
2
Therefore, potassium is correct.

FINAL ANSWER: 2KI(aq) + Pb(NO3)2(aq) PbI2(ppt) + 2KNO3(aq)

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