Question

Calculate the moles of solute dissolved in 4.2 dm3 of a 0.5 mol / dm3 solution.

Answer 1

To find the moles of solute, multiply the solution's volume (4.2 dm³) by its concentration (0.5 mol/dm³), resulting in 2.1 moles of solute.

Step-by-step explanation:

To calculate the moles of solute dissolved in a solution, you multiply the volume of the solution by its concentration. Given a solution volume of 4.2 dm³ and a concentration of 0.5 mol / dm³, you can calculate the moles of solute by multiplying these two values together:

Moles of solute = Volume × Concentration

Moles of solute = 4.2 dm³ × 0.5 mol / dm³

Moles of solute = 2.1 mol

This calculation shows that there are 2.1 moles of solute in the 4.2 dm³ solution.

Answer 2

Example

0.5 mol of sodium hydroxide is dissolved in 2 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed.

Concentration =

Concentration = 0.25 mol/dm3

Volume units

Volumes used in concentration calculations must be in dm3, not in cm3. It is useful to know that 1 dm3 = 1000 cm3. This means:

divide by 1000 to convert from cm3 to dm3

multiply by 1000 to convert from dm3 to cm3

For example, 250 cm3 is 0.25 dm3 (250 ÷ 1000). It is often easiest to convert from cm3 to dm3 before continuing with a concentration calculation.

Question

100 cm3 of dilute hydrochloric acid contains 0.02 mol of dissolved hydrogen chloride. Calculate the concentration of the acid in mol/dm3.

Reveal answer

Converting between units

The relative formula mass of the solute is used to convert between mol/dm3 and g/dm3:

to convert from mol/dm3 to g/dm3, multiply by the relative formula mass

to convert from g/dm3 to mol/dm3, divide by the relative formula mass

Remember: the molar mass is the Ar or Mr in grams per mol.

Example

Calculate the concentration of 0.1 mol/dm3 sodium hydroxide solution in g/dm3. (Mr of NaOH = 40)

Concentration = 0.1 × 40

= 4 g/dm3