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Molarity

Key Stage 3

Meaning

Molarity is aunit of concentration of solutions.

About Molarity

Molarity is shortened with an upper case M.
When molarity is used to describe a solution it may be referred to as molar, eg. a 0.5 molar solution of Sodium Chloride.
Bottles containing acid and alkali usually have their Molarity stated on the side. The more concentrated the higher the molarity.
The molarity of Stomach Acid is around 0.16 Molar (0.16M).
The molarity of Battery Acid can be as much as 5.2 Molar (5.2M).

Key Stage 4

Meaning

Molarity is a unit of concentration of solutions in mol/dm3.

About Molarity

Molarity is shortened with an upper case M.
Molarity is the ratio of the number of moles of a solute to the volume of solvent.
When molarity is used to describe a solution it may be referred to as molar, eg. a 0.5 molar solution of Sodium Chloride.
Bottles containing acid and alkali usually have their Molarity stated on the side. The more concentrated the higher the molarity.

Equation

concentration (mol/dm3) = \(\frac{Moles (mol)}{volume (dm^3)}\)

Where:

Moles = The number of moles of the solute. Found by dividing the mass in grams by the relative formula mass.
volume = The volume of solvent.

Calculating Molarity

117g of NaCl is dissolved in 0.5dm3 of water. Calculate the concentration in g/dm3. 28g of KOH is dissolved in 100ml of water. Calculate the concentration in g/dm3. 7.3g of HCl is dissolved in 400ml of water. Calculate the concentration in g/dm3.

Find the number of moles. mass = 117g

Relative Formula Mass = 58.5g

Number of Moles of a Compound = (Mass of compound)/(Relative Formula Mass of compound)

Number of Moles = \({\frac{m}{M_r}}\)

Number of Moles = \({\frac{117}{58.5}}\)

Number of Moles = 2mol


State the mass in grams and the volume in dm3: Find the number of moles. mass = 28g

Relative Formula Mass = 56g

Number of Moles of a Compound = (Mass of compound)/(Relative Formula Mass of compound)

Number of Moles = \({\frac{m}{M_r}}\)

Number of Moles = \({\frac{28}{56}}\)

Number of Moles = 0.5mol

Find the number of moles. mass = 7.3g

Relative Formula Mass = 36.5g

Number of Moles of a Compound = (Mass of compound)/(Relative Formula Mass of compound)

Number of Moles = \({\frac{m}{M_r}}\)

Number of Moles = \({\frac{7.3}{36.5}}\)

Number of Moles = 0.2mol

volume = 0.5dm3

volume = 100ml

The volume must be converted into dm3 from ml.

volume in dm3 = (volume in ml)/1000

volume = 0.1dm3

volume = 400ml

The volume must be converted into dm3 from ml.

volume in dm3 = (volume in ml)/1000

volume = 0.4dm3

Calculate the Concentration:

concentration (mol/dm3) = \(\frac{Moles (mol)}{volume (dm^3)}\)

concentration = \(\frac{2}{0.5}\)

concentration = 4mol/dm3 = 4M

Calculate the Concentration:

concentration (mol/dm3) = \(\frac{Moles (mol)}{volume (dm^3)}\)

concentration = \(\frac{0.5}{0.1}\)

concentration = 5mol/dm3 = 5M

Calculate the Concentration:

concentration (mol/dm3) = \(\frac{mass (g)}{volume (dm^3)}\)

concentration = \(\frac{0.2}{0.4}\)

concentration = 0.5mol/dm3 = 5M