Kc Units and Values - Equilibria Pt A - (4.3)

Question 1

What does M also stand for?

Answer 1

moldm^-3

Question 2

If there are 345cm^3 of a gas in a 1L container, how do you work out the concentration of this gas?

Answer 2

Molar volume = 24dm^3 (of any gas)
345cm^3 = 0.345dm^3
0.345/24 = 0.0144 mol per 1L
1L = 1dm^3
Therefore, the concentration is 0.0144 moldm^-3

Question 3

If you are given the concentration of the product at equilibrium, how do you work out the concentration of the reactants at equilibrium?
E.g at equilibrium there is 0.4M of product
& the initial concentration of reactants is 0.3M
Equation: A+B -> 2C

Answer 3

Concentration of products:

• For the initial concentration it should be 0 since it is a product & it hasn’t been made yet
• From initial to equilibrium it has increased by +0.4

Concentration of reactants:

• Therefore, the conc. of reactants at equilibrium should decrease (as they are used to make products)
• The mole ratio of C is double A & B so the conc. of reactants will decrease by 0.4/2 = 0.2
• So at equilibrium it is 0.3-0.2 = 0.1M

Question 4

How do you work out the units for Kc?

Answer 4

units of products all multiplied/units of all reactants multiplied
(include if conc. have powers)
e.g if Kc = [B]/[A]^2, the units are M/M^2 = M^-1 = 1/moldm^-3 = mol^-1dm^3