0.9 Quantitative aspects of chemical change  (Page 5/10)

Group work : understanding moles, molecules and avogadro's number

Divide into groups of three and spend about 20 minutes answering the following questions together:

1. What are the units of the mole? Hint: Check the definition of the mole.
2. You have a $56\mathrm{g}$ sample of iron sulphide ( $\mathrm{FeS}$ )
   1. How many moles of $\mathrm{FeS}$ are there in the sample?
   2. How many molecules of $\mathrm{FeS}$ are there in the sample?
   3. What is the difference between a mole and a molecule?
3. The exact size of Avogadro's number is sometimes difficult to imagine.
   1. Write down Avogadro's number without using scientific notation.
   2. How long would it take to count to Avogadro's number? You can assume that you can count two numbers in each second.

More advanced calculations

1. Calculate the molar mass of the following chemical compounds:
   1. $\mathrm{KOH}$
   2. $\mathrm{FeCl}{}_3$
   3. ${\mathrm{Mg(OH)}}_2$
2. How many moles are present in:
   1. $10\mathrm{g}$ of $\mathrm{Na}{}_2$ SO ${}_4$
   2. $34\mathrm{g}$ of $\mathrm{Ca(OH)}{}_2$
   3. $2,45\times 10{}^{23}$ molecules of $\mathrm{CH}{}_4$ ?
3. For a sample of $\mathrm{0,2}\mathrm{moles}$ of potassium bromide ( $\mathrm{KBr}$ ), calculate...
   1. the number of moles of ${\mathrm{K}}^{+}$ ions
   2. the number of moles of ${\mathrm{Br}}^{-}$ ions
4. You have a sample containing $3\mathrm{moles}$ of calcium chloride.
   1. What is the chemical formula of calcium chloride?
   2. How many calcium atoms are in the sample?
5. Calculate the mass of:
   1. $3\mathrm{moles}$ of $\mathrm{NH}{}_4\mathrm{OH}$
   2. $\mathrm{4,2}\mathrm{moles}$ of $\mathrm{Ca}\left(\mathrm{NO}{}_3\right){}_2$
6. $\mathrm{96,2}\mathrm{g}$ sulphur reacts with an unknown quantity of zinc according to the following equation: $\mathrm{Zn}+\mathrm{S}\to \mathrm{ZnS}$
   1. What mass of zinc will you need for the reaction, if all the sulphur is to be used up?
   2. What mass of zinc sulphide will this reaction produce?
7. Calcium chloride reacts with carbonic acid to produce calcium carbonate and hydrochloric acid according to the following equation: ${\mathrm{CaCl}}_2+{\mathrm{H}}_2{\mathrm{CO}}_3\to {\mathrm{CaCO}}_3+2\mathrm{HCl}$ If you want to produce $10\mathrm{g}$ of calcium carbonate through this chemical reaction, what quantity (in g) of calcium chloride will you need at the start of the reaction?
   

The composition of substances

The empirical formula of a chemical compound is a simple expression of the relative number of each type of atom in that compound. In contrast, the molecular formula of a chemical compound gives the actual number of atoms of each element found in a molecule of that compound.

Empirical formula

The empirical formula of a chemical compound gives the relative number of each type of atom in that compound.

Molecular formula

The molecular formula of a chemical compound gives the exact number of atoms of each element in one molecule of that compound.

The compound ethanoic acid for example, has the molecular formula $\mathrm{CH}{}_3\mathrm{COOH}$ or simply $\mathrm{C}{}_2\mathrm{H}{}_4\mathrm{O}{}_2$ . In one molecule of this acid, there are two carbon atoms, four hydrogen atoms and two oxygen atoms. The ratio of atoms in the compound is 2:4:2, which can be simplified to 1:2:1. Therefore, the empirical formula for this compound is $\mathrm{CH}{}_2\mathrm{O}$ . The empirical formula contains the smallest whole number ratio of the elements that make up a compound.

Knowing either the empirical or molecular formula of a compound, can help to determine its composition in more detail. The opposite is also true. Knowing the composition of a substance can help you to determine its formula. There are four different types of composition problems that you might come across: