Question

Calculate the molarity of a solution made by adding 30.7 mL of concentrated potassium hydroxide (34.5 wt%, density 1.46 g/mL) to some water in a volumetric flask, then adding water to the mark to make exactly 200 mL of solution. (It is important to add concentrated acid or base to water, rather than the other way, to minimize splashing and maximize safety.)

Answer 1

volume added = 30.7 mL

density = 1.46 g/mL

So,

mass of solution = density * volume

= 1.46 g/mL * 30.7 mL

= 44.8 g

mass of KOH added = 34.5 % of total mass

= 34.5 * 44.8 / 100

= 15.5 g

Molar mass of KOH,

MM = 1*MM(K) + 1*MM(O) + 1*MM(H)

= 1*39.1 + 1*16.0 + 1*1.008

= 56.108 g/mol

number of mol of KOH,

n = mass of KOH/molar mass of KOH

=(15.5 g)/(56.108 g/mol)

= 0.2763 mol

final volume , V = 200 mL

= 0.200 L

Molarity,

M = number of mol / volume in L

= 0.2763/0.200

= 1.38 M

Answer: 1.38 M