Concentration of Solutions Flashcards

All 11 cards in this deck

What is the formula for concentration in mol/dm$^3$?

$\text{concentration} = \dfrac{\text{amount of solute (mol)}}{\text{volume of solution (dm}^3)}$

How do you convert a volume in cm$^3$ to dm$^3$?

e.g. $25.0$ cm$^3$

Divide by $1000$.
$25.0 \div 1000 = 0.0250$ dm$^3$

What are the steps to find the amount in moles of solute in a solution?

e.g. 500 cm$^3$ of 0.2 mol/dm$^3$ NaOH

1. Convert volume to dm$^3$: $500 \div 1000 = 0.5$ dm$^3$
2. Multiply volume by concentration: $0.5 \times 0.2 = 0.1$ mol

What are the steps to calculate the mass of solute needed to make a stated volume of solution of a given concentration?

e.g. 250 cm$^3$ of 0.1 mol/dm$^3$ NaOH ($M_r = 40$)

1. Volume in dm$^3$: $250 \div 1000 = 0.25$ dm$^3$
2. Moles $=$ volume $\times$ concentration $= 0.25 \times 0.1 = 0.025$ mol
3. Mass $=$ moles $\times M_r = 0.025 \times 40 = 1.0$ g

How do you convert a concentration in mol/dm$^3$ to one in g/dm$^3$?

e.g. 0.5 mol/dm$^3$ NaOH ($M_r = 40$)

Multiply by the relative formula mass.
$0.5 \times 40 = 20$ g/dm$^3$

What are the steps to find the concentration of a solution from titration results?

e.g. 20 cm$^3$ of NaOH neutralised by 10 cm$^3$ of 0.1 mol/dm$^3$ $\mathrm{H_2SO_4}$; $2\mathrm{NaOH} + \mathrm{H_2SO_4} \rightarrow \mathrm{Na_2SO_4} + 2\mathrm{H_2O}$

1. Moles of known solution $= \frac{10}{1000} \times 0.1 = 0.001$ mol
2. Use the equation ratio: moles NaOH $= 2 \times 0.001 = 0.002$ mol
3. Divide by its volume in dm$^3$: $0.002 \div 0.020 = 0.1$ mol/dm$^3$

True or false? Adding more water to a solution, with no extra solute, decreases its concentration in mol/dm$^3$.

True. The same amount of solute is spread through a larger volume of solution.

True or false? Doubling the mass of solute dissolved and halving the volume of the solution multiplies the concentration by 4.

True. Concentration is proportional to solute amount and inversely proportional to volume.

True or false? A volume of 25 cm$^3$ can be used directly in the equation concentration $=$ moles $\div$ volume.

False. The volume must first be converted to dm$^3$ by dividing by 1000.

True or false? In a titration calculation you can assume the two solutions react in a 1 : 1 mole ratio.

False. You must use the ratio given by the balanced symbol equation.

True or false? Rounding the number of moles part-way through a concentration calculation can make the final answer wrong.

True. Keep the unrounded values in the calculator and round only the final concentration.