Moles: Concentration and titration calculations

Understanding how to use moles to find the concentration of solutions, and how to deal with titration calculations.

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2.00 moles of sodium chloride is dissolved in water and made up to 250cm3 of solution. What is the concentration of the solution (mol dm−3)?

Concentration (mol dm−3) = moles (mol) / volume of solution (dm3)

Concentration = 2.00 / 0.250 = 8.00 mol dm−3

15.0g of magnesium chloride is dissolved in water and made up to 250cm3 of solution. What is the concentration of the solution (mol dm−3) to 3 sf?

Moles = mass / molar mass = 15.0 / 95.21 (MgCl2) = 0.1575464 mol

Concentration (mol dm−3) = moles (mol) / volume of solution (dm3)

Concentration = 0.1575464 / 0.250 = 0.6301859 mol dm−3 = 0.630 mol dm−3 (3sf)

How many moles (3sf) of sodium hydroxide is dissolved in 500cm3 of a solution of concentration 0.800 mol dm−3?

Moles (mol) = concentration (mol dm−3) × volume of solution (dm3)

Moles = 0.800 × 0.500 = 0.400 mol (3sf)

What mass, in g (3sf), of potassium hydroxide is dissolved in 250cm3 of a solution of concentration 0.600 mol dm−3?

Moles (mol) = concentration (mol dm−3) × volume of solution (dm3)

Moles = 0.600 × 0.250 = 0.150 mol

Mass = moles × molar mass = 0.150 × 56.11 (KOH) = 8.4165 = 8.42g (3sf)

A student takes 150 cm3 of a 1.75 mol dm−3 aqueous solution and dilutes it with water to 500cm3 in a volumetric flask. What is the concentration of the new solution (mol dm−3) to 3sf?

Moles (mol) = concentration (mol dm−3) × volume of solution (dm3)

Moles in the sample of original solution = 1.75 × 0.150 = 0.2625 mol

Concentration (mol dm−3) = moles (mol) / volume of solution (dm3)

Concentration of the new solution = 0.2625 / 0.500 = 0.525 mol dm−3

0.450 moles of sodium hydroxide is dissolved in water and made up to 750cm3 of solution. What is the concentration of the solution in g dm−3 (3sf)?

Concentration (mol dm−3) = moles (mol) / volume of solution (dm3)

Concentration = 0.450 / 0.750 = 0.600 mol dm−3

Converting moles to mass: mass = moles × molar mass = 0.600 × 40.00 = 24.0g

Concentration = 24.0 g dm−3

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