Question

What is the molarity (molar concentration, unit = M) of K+ found in 200 mL 0.2 M K2HPO4 solution?

Answer 1

0.4 M

Step-by-step explanation:

The process that takes place in an aqueous K₂HPO₄ solution is:

• K₂HPO₄ → 2K⁺ + HPO₄⁻²

First we calculate how many K₂HPO₄ moles are there in 200 mL of a 0.2 M solution:

• 200 mL * 0.2 M = 40 mmol K₂HPO₄

Then we convert K₂HPO₄ moles into K⁺ moles, using the stoichiometric coefficients of the reaction above:

• 40 mmol K₂HPO₄ *
  `(2mmolK^+)/(1mmolK_2HPO_4)` = 80 mmol K⁺

Finally we divide the number of K⁺ moles by the volume, to calculate the molarity:

• 80 mmol K⁺ / 200 mL = 0.4 M