Calculating concentrations - Higher
It is often more useful to know the concentrationA measure of the mass or amount of solute dissolved in a given volume of solvent or solution. of a reactantA substance that reacts together with another substance to form products during a chemical reaction. in moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 脳10虏鲁 number of particles)./dm3 as this relates more easily to reacting ratios.
Concentration using moles
The concentration of a solution can be calculated using:
- the number of moles of dissolvedA substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution.soluteThe dissolved substance in a solution., mol
- the volume of solution (or solventThe liquid in which the solute dissolves to form a solution.) in cubic decimetres, dm3
The units for concentration are also shown as mol dm-3 or M (for 鈥榤olar鈥), but this means the same as moles/dm3.
Question
0.5 moles of sodium hydroxide is dissolved in 2 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed.
concentration (mol/dm3) = \(\frac{number\ of\ moles\ of\ solute}{volume\ (dm^3)}\)
concentration = \(\frac{0.5\ moles}{2\ dm^3}\)
= 0.25 moles/dm3
Question
100 cm3 of dilute hydrochloric acid contains 0.02 moles of dissolved hydrogen chloride. Calculate the concentration of the acid in moles/dm3.
volume of acid in dm3 = 100/1,000 = 0.1 dm3
concentration of acid = mass in g/volume in dm3
= \(\frac{0.02}{0.1}\)
= \(\frac{0.2\ moles}{dm^3}\)
Converting between units
Change mass into moles to convert from g/dm3 to moles per dm3.
Use the relative formula mass Mr to do this:
number of moles = \(\frac{mass}{M_r}\)
Question
Calculate the concentration of 7.3 g/dm3 hydrochloric acid in moles/dm3. (Relative formula mass of HCl = 36.5)
number of moles = \(\frac{mass}{M_r}\)
= \(\frac{7.3}{36.5}\)
= 0.2 mol
So concentration = 0.2 moles/dm3
Converting between units
Change moles into mass to convert from moles/dm3 to g/dm3.
Rearrange the equation:
mass = Mr x number of moles
Question
Calculate the concentration of 0.1 moles/dm3 sodium hydroxide solution in g/dm3. (Relative formula mass: NaOH = 40.0)
mass = Mr x number of moles
= 40.0 脳 0.1
= 4 g
So concentration = 4 g/dm3