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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 7

Lesson 3: Calculating the equilibrium constant

# Worked examples: Calculating equilibrium constants

In this video, we'll calculate equilibrium constants using measurements of concentration and partial pressures at equilibrium. First, we'll find Kc for an equilibrium system using equilibrium concentrations. Then, we'll find Kp for a different system using equilibrium partial pressures. Created by Jay.

## Want to join the conversation?

• Q.
At 400°C a 50L container has 2 mole N2 and 6 mole H2.
If at equilibrium, 2 mole NH3 is produced, what's the value of Kc
• For the last question when finding the Kp I got .28 instead of .11 when I plugged (.2)(3.4)/(3.9)(1.6). I don't know what I did wrong
• The other replier is correct. Computers, like calculators, are stupid so they’ll only know to perform the calculations in the order you input them into the calculator. So if you tell it to do the operation you stated, the calculator will read it as 0.2 x 3.4 ÷ 3.9 x 1.6, and do it in that order (from left to right like PEMDAS). Which is why you get 0.28 instead of the actual answer of 0.11.

You have to specify to the calculator which order you’d like the operations to be done in with parentheses. If you input it as (0.2 x 3.4) ÷ (3.9 x 1.6), you’re telling the calculator you want it to calculate the products in the parentheses first, then divide those two products to get a quotient, which is actually what we want to do. When doing math on a computer you have to be very exact about what you want or it’ll misinterpret it.

Hope that helps.
• At , what would happen if the mole ration was not 1-1?
(1 vote)
• The change corresponds to their coefficients in the chemical equation. If CO has a 2 coefficient, and water still had a 1, the ratio would be 2:1. This means water would increase by x amount, but CO would increase by 2x amount since it forms at twice the rate that water does.

Hope that helps.