1.3 Reacting masses and volumes

Nature of science:

Making careful observations and obtaining evidence for scientific theories—Avogadro's initial hypothesis.

Understandings:

• Reactants can be either limiting or excess.
• The experimental yield can  be different from the theoretical yield. Avogadro’s law enables the mole ratio of reacting gases to be determined from volumes of the gases.
• The molar volume of an ideal gas is a constant at specified temperature and pressure.
• The molar concentration of a solution is determined by the amount of solute and  the volume of solution.
• A standard solution is one  of known concentration.

Applications and skills:

• Solution of problems relating to reacting quantities, limiting and excess reactants, theoretical, experimental and percentage yields.
• Calculation of reacting volumes of gases using Avogadro’s law. Solution of problems and analysis of graphs involving the relationship between temperature, pressure and  volume for a fixed mass of an ideal gas.
• Solution of problems relating to the ideal gas equation.
• Explanation of the deviation of real gases from ideal behaviour at low temperature and high pressure.
• Obtaining and  using experimental values to calculate the molar mass of a gas from the ideal gas equation.
• Solution of problems involving molar concentration, amount of solute and volume of solution.
• Use of the experimental method of titration to calculate the concentration of a solution by reference to a standard solution.

Guidance:

• Values for the molar volume of an ideal gas are  given in the data  booklet in section 2.
• The ideal gas equation, PV = nRT, and  the value of the gas constant (R) are given in the data  booklet in sections 1 and 2.
• Units of concentration to include: g dm-3, mol dm-3  and  parts per million (ppm).

Aims:

• Aim 6: Experimental design could include excess and limiting reactants. Experiments could include gravimetric determination by precipitation of an insoluble salt.
• Aim 7: Data loggers can be used to measure temperature, pressure and volume changes in reactions or to determine the value of the gas constant, R.
• Aim 8: The unit parts per million, ppm, is commonly used in measuring small levels of pollutants in fluids. This unit is convenient for communicating very low concentrations, but is not a formal SI unit.

International-mindedness:

The SI unit of pressure is the Pascal (Pa),  N m^-2, but many other units remain in common usage in different countries. These include atmosphere (atm), millimetres of mercury  (mm Hg), Torr, bar and  pounds per square inch (psi).The bar (10⁵ Pa) is now widely used as a convenient unit, as it is very close to 1 atm. The SI unit for volume is m3, although litre is a commonly used unit.