All courses › Chemistry 2 › An unknown concentration

An unknown concentration

If you know K and every concentration but one, you can solve the equation for the missing one. For the reaction between hydrogen and iodine, HI is squared, so you must take the square root at the end. Remember that a concentration cannot be negative, so it is the positive root that applies. The units are moles per litre when K is calculated from concentrations. Put the answer back into the fraction and check that you get the same K.

[HI]=K[H2][I2][\mathrm{HI}] = \sqrt{K[\mathrm{H}_2][\mathrm{I}_2]}when the others are known

Symbols

KKequilibrium constant, here 8
[HI][\mathrm{HI}]the unknownmol/L

Example

K = 8, [H2] = 0.50 mol/L and [I2] = 0.50 mol/L.

[HI]=8⋅0.50⋅0.50≈1.414[\mathrm{HI}] = \sqrt{8 \cdot 0.50 \cdot 0.50} \approx 1.414 mol/L.

Check: 1.414 squared, divided by 0.50 times 0.50, should give about 8.
Practise chemical equilibrium for free →

← Reaction quotient · Alkanes →

Part of Chemistry 2: Chemical equilibrium.