Calculating Concentration (Cambridge (CIE) O Level Chemistry): Revision Note

A student dissolved 10 g of sodium hydroxide, NaOH, in 2 dm³ of distilled water. Calculate the concentration of the solution in g/dm³.

Answer

Step 1: State the known values

• Mass = 10 g

• Volume of solution = 2 dm³

Step 2: Substitute the values into the concentration equation

concentration = $\frac{\mathrm{mass}\mathrm{of}\mathrm{solute}\left(\mathrm{g}\right)}{\mathrm{volume}\mathrm{of}\mathrm{solution}\left({\mathrm{dm}}^3\right)}$

concentration = $\frac{10\mathrm{g}}{2{\mathrm{dm}}^3}$

concentration = 5 g/dm³

Calculate the amount of solute, in moles, present in 2.5 dm³ of a solution whose concentration is 0.2 mol/dm³.

Answer

Step 1: State the known values

• Concentration of solution = 0.2 mol/dm³

• Volume of solution = 2.5 dm³

Step 2: Rearrange the concentration equation

concentration = $\frac{\mathrm{moles}}{\mathrm{volume}\mathrm{of}\mathrm{solution}}$

moles = concentration x volume

Step 3: Substitute the values into the rearranged equation and solve for moles

moles = 0.2 x 2.5 = 0.5 moles

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol/dm³, when 80 g is dissolved in 500 cm³ of water.

Relative atomic masses, A_r:  Na= 23;   H= 1;   O= 16

Answer

Step 1: Calculate the M_r of NaOH

M_r of NaOH = 23 + 16 + 1 = 40 g

Step 2: Convert the mass of NaOH to moles

40 g = 1 mole

80 g = 2 moles

Step 3: Convert cm³ to dm³ for H₂O

$\frac{500}{1000}$ = 0.5 dm³

Step 4: Substitute the values into the equation and solve for concentration

concentration = $\frac{2}{0.5}$ = 4 mol/dm³

25.0 cm³ of 0.050 mol/dm³ sodium carbonate was completely neutralised by 20.0 cm³ of dilute hydrochloric acid. Calculate the concentration in mol/dm³ of the hydrochloric acid.

Answer

Step 1: Convert the volume of sodium carbonate solution from cm³ to dm³

$\frac{25.0}{1000}$ = 0.025 dm³

Step 2: Rearrange the concentration equation

concentration = $\frac{\mathrm{moles}}{\mathrm{volume}\mathrm{of}\mathrm{solution}}$

moles = concentration x volume

Step 3: Substitute the values into the rearranged equation and solve for moles

moles = 0.050 x 0.025 = 0.00125 mol

Step 4: Calculate the moles of hydrochloric acid reacted

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

1 mole of Na₂CO₃ reacts with 2 moles of HCl

0.00125 mole of Na₂CO₃ reacts with 0.00250 moles of HCl

Step 5: Convert the volume of hydrochloric acid from cm³ to dm³

$\frac{20.0}{1000}$ = 0.02 dm³

Step 6: Calculate the concentration of hydrochloric acid

concentration = $\frac{\mathrm{moles}}{\mathrm{volume}\mathrm{of}\mathrm{solution}}$

concentration = $\frac{0.00250}{0.02}$

concentration = 0.125 mol/dm³

Remember to always convert the units from cm³ to dm³ by dividing by 1000.