Quantative Chemistry

You should be able to calculate the masses of reactants and products from balanced equations, and the percentage composition by mass of an element in a compound.

Percentage composition Percentage composition is just a way to describe what proportions of the different elements there are in a compound.If you have the formula of a compound, you should be able to work out the percentage by mass of an element in it.

Example The formula for sodium hydroxide is NaOH. It contains three different elements:Na, O and H. But the percentage by mass of each element is not simply 33.3 per cent, because each element has a different relative atomic mass. You need to use the Ar values to work out the percentages. Here is how to do it: What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)? First, work out the relative formula mass of the compound, using the Ar values for each element. In the case of sodium hydroxide, these are Na = 23, O = 16, H = 1. (You will be given these numbers in the exam.) Next, divide the Ar of oxygen by the Mr of NaOH, and multiply by 100 to get a percentage. Mr of NaOH is 23 + 16 + 1 = 40(16 ÷ 40 ) × 100 = 0.4 × 100 = 40%So the percentage by mass of oxygen in sodium hydroxide is 40%.

Conservation of mass Mass is never lost or gained in chemical reactions. We say that mass is conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginningThis fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:example:   Calcium carbonate   ->   Calcium oxide   +   Carbon dioxide            50g                               22g                        28g

Percentage YieldThe principle of conservation of mass lets you calculate the theoretical mass of product expected in a chemical reaction.However, it is not always possible in practice to get the entire calculated amount of product. This is because: Reversible reactions may not go to completion Some product may be lost when it is removed from the reaction mixture Some of the reactants may react in an unexpected way

Yield The yield of a reaction is the mass of product obtained: The theoretical yield is the maximum theoretical mass of product in a reaction (calculated using the idea of conservation of mass) The actual yield is the mass of product you get when you actually do the reaction The percentage yield is the ratio of actual mass of products obtained compared with the maximum theoretical mass.

Reversible reactions Many reactions, such as burning fuel, are irreversible - they go to completion and cannot be reversed easily.Reversible reactions are different. In a reversible reaction, the products can react to produce the original reactants again.

When writing chemical equations for reversible reactions, we do not use the usual one-way arrow. Instead, we use two arrows, each with just half an arrowhead - the top one pointing right, and the bottom one pointing left. For example:ammonium chlorideammonia + hydrogen chloride

Quantative Chemistry

Conservation of mass

Percentage Yield

Reversible Reactions