6.1 Stoichiometry and composition  (Page 4/5)

Molarity and the concentration of solutions

1. 5.95 g of potassium bromide was dissolved in 400 cm ${}^3$ of water. Calculate its molarity.
2. 100 g of sodium chloride (NaCl) is dissolved in 450 cm ${}^3$ of water.
   1. How many moles of NaCl are present in solution?
   2. What is the volume of water (in dm ${}^3$ )?
   3. Calculate the concentration of the solution.
   4. What mass of sodium chloride would need to be added for the concentration to become 5.7 mol.dm ${}^{-3}$ ?
3. What is the molarity of the solution formed by dissolving 80 g of sodium hydroxide (NaOH) in 500 cm ${}^3$ of water?
4. What mass (g) of hydrogen chloride (HCl) is needed to make up 1000 cm ${}^3$ of a solution of concentration 1 mol.dm ${}^{-3}$ ?
5. How many moles of H ${}_2$ SO ${}_4$ are there in 250 cm ${}^3$ of a 0.8M sulphuric acid solution? What mass of acid is in this solution?

Stoichiometric calculations

Stoichiometry is the calculation of the quantities of reactants and products in chemical reactions. It is also the numerical relationship between reactants and products. The Grade 10 discussion on representing chemical changeshowed how to write balanced chemical equations. By knowing the ratios of substances in a reaction, it is possible to use stoichiometry to calculate the amount of either reactants or products that are involved in the reaction. The examples shown below will make this concept clearer.