Solution of simple numerical problems in chemistry

Working out of simple problems in chemistry can be reduced to three simple steps. Consider the following question:

Hydrochloric acid is neutralised by sodium hydroxide according to the following equation:
HCl + NaOH ® H2O + NaCl
An experimenter starts off with 100kg of hydrochloric acid.

How much sodium hydroxide is required to neutralise 100kg of hydrochloric acid ?

  1. The first step consists in balancing the equation involved. This step is unnecessary in this particular case, since the equation is already balanced. In this case, simply copy down the equation in the form given.

    2. HCl + NaOH ® H2O + NaCl

  2. Step 2 involves calculating the relative atomic mass (RAM) of each term in the equation. In this case, there are four different terms, meaning that four different calculations are required. The mass number of each of the atoms in the equation may be obtained from a periodic table. In examination questions, any mass numbers required are usually specified, such that you would not need to resort to a periodic table.. In this case, the following relative atomic masses are given (units are g mol-1): H = 1.0, Cl = 35.5, Na = 23.0, O = 16.0
    The four RAMs required are therefore calculated as follows:
    HCl = H + Cl = 1 + 35.5 = 36.5
    NaOH = Na + O + H = 23 + 16 + 1 = 40
    H2O = H + H + O = 1 + 1 + 16 = 18
    NaCl = Na + Cl = 23 + 35.5 = 58.5
    write the RAMs underneath each term of the equation as follows:

    HCl + NaOH ® H2O + NaCl
    36.5 40 18 58.5
  3. Step 3 starts as follows: Identify the quantity you are being requested to calculate. In this case, you are requested to calculate the amount of sodium hydroxide (NaOH). So place an interrogation mark below NaOH as follows:
    HCl + NaOH ® H2O + NaCl
    36.5 40 18 58.5
    ?

    Which quantity are we given ? In this case, Hydrochloric acid (HCl), of which the experimenter uses 100kg. So list 100kg (the quantity given) below the term HCl, as follows:
    HCl + NaOH ® H2O + NaCl
    36.5 40 18 58.5
    100Kg ?

    You will notice that some terms now consist of three "layers" whilst others only have two layers. Delete the terms with only two layers (in this case, H2O and NaCl). They will play no further part in the equation:

    HCl + NaOH
    36.5 40
    100kg ?

    Calculate the missing quantity (mass of NaOH required, denoted by the interrogation mark) by simple proportion: (100kg x 40)¸ 36.5
    the answer should work out to 109.6 kg.
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