Question

A stock bottle of an acid reads: 56% by mass and 1.25 specific gravity. If the molar mass of the acid is 70g, find the mole per cubic decimeter

Answer 1

Answer:

• 10 mol / dm³

Explanation:

Cubic decimeter is the same unit as liter; so, mole per cubic decimeter is mole per liter, and that is the unit of concentration of molarity. Thus, what is asked is the molarity of the solution. This is how you find it.

1. Take a basis: 1 dm³ = 1 liter = 1,000 ml

2. Calculate the mass of 1 liter (1,000 ml) of solution:

• density = mass / volume ⇒ mass = density × volume

Here, the density is given through the specific gravity

Scpecific gravity = density of acid / density of water

Take density of water as 1.00 g/ml.

• density of solution = 1.25 g/ml

• mass solution = 1.25 g/ml × 1,000 ml = 1,250 g

3. Calculate the mass of solute (pure acid)

• % m/m = (mass of solute / mass of solution) × 100

• 56 = mass of solute / 1,250 g × 100

• mass of solute = 56 × 1,250g / 100 = 700 g

4. Calculate the number of moles of solute:

• moles = mass in grams / molar mass = 700 g / 70 g/mol = 10 mol

5. Calculate molarity (mol / dm³)

• M = number of moles of solute / liter of solution = 10 mol / 1 liter = 10 mol/liter.