Calculations in Chemistry

How heavy are atoms? This is common question but almost impossibly difficult to accurately give values in Kg that we can visualise. Instead, in chemistry we focus on the relative masses of atoms based on the protons and neutrons in their nuclei. We do not count electrons as they have such a tiny mass.

On the periodic table, the numbers in each element’s box tell us the number of protons and electrons. The biggest number is the “Mass Number” which tells us the number of protons and neutrons added together and the smallest number is the “Atomic Number” which tells us the number of protons in the nucleus. From this, we can work out:


The number of neutrons = mass number - atomic number.

MOLES: We, as chemists, use the word moles to show these proportions regularly, such as 2 moles of oxygen. Do not be frightened by the complexity of this term, it is, simply put, the same as a dozen. If you buy a dozen eggs, you are getting 12 eggs, if you buy a mole of carbon-12, you are buying 12g of carbon-12 or 602000000000000000000000 atoms of Carbon-12. (The number of atoms in a mole is called Avagadro's constant and it written 6.02x1023)

In a reaction, mass is always conserved, if you add 5g of one reactant and 4g of another, you will produce 9g of other substances. You can calculate this using moles. Follow these simple steps every time:
HOW MANY MOLES KNOWN (you will be given a mass, divide it by the formula mass of that substance;
RATIO (using the equation, check the ratio of substance known to substance you are finding out, this is usually the same unless a higher question);
MOLES UNKNOWN (using the ratio, you may need to double or halve the number of moles of the mystery substance);
MASS UNKNOWN (simply multiply the number of moles by the formula mass of this new substance).

If you have a solid dissolved into water, you can calculate its concentration. Rather than the simple units of mass per litre, we as chemists, also use moles per dm3. To find this out, simply find out the mass of substance and divide it by its formula mass then divide this by the volume of water in dm3 and you have the answer.

If we are working with gases, we can find out the number of moles not just by their mass, which is tricky for a gas, we can also do it using volume. Simply put, you just need to know that 1 mole of gas at room temperature and pressure occupies 24dm3.

Now that we can work out the atomic and formula masses, we can work out the percentage an element is in a compound. If we take Sodium Fluoride, you cannot say it it 50:50 as there is one of each, you want to know what percentage of this mass is Sodium. We work this out by dividing the atomic mass of sodium by the combined formula mass.

This is 23/(23+19) x100 = 54.8% of the mass is Sodium.

Percentage yield = Mass of product produced (g)              x100
                           Maximum possible amount of product (g)

This is used to measure how successful your reaction is. In industry, this is particularly important as it relates to profit. Industries also favour continuous processes rather than batch ones to again improve productivity and efficiency while reducing the waste that will lower the yield.

You will later study about the Haber process and see that it is a reversible reaction. Any reaction that can turn its reactants into products and its products into reactants is a reversible reaction. Certain conditions will favour wither the forward or the reverse reaction. A catalyst will not shift the equilibrium either way, however, it will make a reaction reach its point of equilibrium faster.

Key words and terms for this topic: mass number, atomic mass, relative atomic mass, relative formula mass, isotope, mole, empirical formula, molecular formula, percentage mass, percentage yield, reversible reaction, chromatography, gas chromatography, mass spectrometry, molecular ion peak, GCMS.

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