Question

What is the molarity of a solution made by dissolving 5.25 g of potassium
hydrogen phthalate in distilled water and making it up to 250 mL with
distilled water in a volumetric flask?

Answer 1

First of all find the amount of potassium hydrogen phthalate:

$$\nu = m/M$$

Find molar mass of potassium hydrogen phthalate (KHC₈H₄O₄):

$M = 39 + 1 + (8*12) + 4 + (4*16) = 204 (g/mol)$

$\nu = 5.25/204 = 0.0257 (mol)$

Find the molarity of the solution, if there is 0.0257 mol in 250 ml:

$C_m = \nu/V$

$C_m = 0.0257/0.25 = 0.1028 (mol/L)$

Answer: the molarity of the potassium hydrogen phthalate solution is 0.1028 mol/L.