Chemistry Calculations Summary

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					1. Balancing Symbol Equations Background information: Valency and writing chemical formulae The three steps in writing a balanced equation are: 1. Write the word equation - make sure you include all reactants and products 2. Write in the symbols and chemical formulae 3. Balance the equation – check that you have the same number of each type of atom on both sides of the equation. 2. Relative Formula Mass Definition: The sum of the atomic masses of all the atoms in a molecule or compound E.g. 1 Methane (CH4) 1 x C = 1x12 = 12 4xH=4x1= 4 Total = 16 E.g. 2 Sodium sulphate (Na2SO4) 2 x Na = 2 x 23 = 46 1 x S = 1 x 32 = 32 4 x O = 4 x 16 = 16 Total = 142

3. % by Mass Using the following formula we are able to calculate the % mass of an element within a molecule E.g.

% Element


Mass of Element RFM



Calculate the % by mass of Si and O in SiO2 (RFM of SiO2 = 60) % Si %O = = 28 60 32 60 X X 100 100 = = 47% 53%

4. The Mole Definition: A mole is the quantity of a substance in grams which contains 6 x 1023 particles (Avogadro’s constant). These particles could be atoms or molecules.

The mole is an amount of a chemical.
(It is simply a name for a fixed number) Atoms are too small to be weighed or counted individually One mole of an element weighs the Relative Atomic Mass (RAM) in grams.

One mole of a compound (or an element that exists as molecules, e.g. diatomic molecules) weighs the Relative Formula Mass (RFM or relative molecular mass, RMM) in grams Relative Formula / Molecular / Molar Mass = sum of all the relative atomic masses of all the atoms in a formula/molecule of a substance. It has no units Molar Mass = Mass of one mole of an element or compound. Units= g mol -1 5. Equations involving the mole. 1. A mole of a substance can be calculated from the following equation: No. of moles = Mass of substance (in grams) Relative Molecular Mass (r.f.m)

Mass Moles RFM

2. If the substance is a gas then the number of moles can also be calculated from the volume of the gas: No. of moles = Volume of gas in dm3 Molar Gas Volume in dm3

The Molar Gas Volume depends on the temperature and pressure of the system. At room temperature and pressure (R.T.P.) The molar gas volume = 24dm3 3. If given a solution of a defined concentration then the number of moles can be calculated from No. of moles = Volume in cm3 x 1000 Concentration (moles/dm3)

If the methods above are used sensibly and the equation is balanced carefully there should be no problem with calculations.

6. Calculations of Reacting Masses Rules: i) Write a balanced symbol equation ii) Write the ratio of what is required above the equation iii) Convert the quantity you know into moles iv) Carry this across the equation according to the ratio worked out v) Reconvert moles back to mass. Example: Calculate the mass of carbon needed to reduce 15.9g copper (II) oxide to copper by the reaction CuO(s) + C(s)  Cu(s) + CO(g)

7. Empirical and Molecular Formula The empirical formula gives the simplest whole number ratio of the atoms of each element to each other in the compound. To find the empirical formula of a compound we first calculate the number of moles of each substance present in a sample and then calculate the simplest whole number ratio. Steps to follow: 1) Put symbols Left  Right 2) Put mass of each below. 3) Convert mass to number of moles 4) Divide through by the smallest number of moles 5) Ratio gives empirical formula Example : An organic compound was analysed and found to contain by mass Carbon 48.8% Hydrogen 13.5% Nitrogen 37.7% What is its empirical formula?

8. Using moles in solution calculations (i.e. Determination of concentration) Concentration: the number of moles of solute in 1dm3 of solution. (units = mol dm-3) (it may also refer to the mass of solute in 1dm3 of solution, units are g dm-3)

mol. dm-3 x RFM  RFM g dm-3
These calculations are almost exactly the same as predicting mass changes, except that the equation used to convert into moles and re-convert back into moles is different. The same basic principles apply. Number of moles = Volume (cm3) x Molarity (mol/dm3) 1000 Example 1: For the reaction NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l) 25cm3 of xM NaOH is put into a conical flask and it requires 22.5 cm 3 0.1M HCl for neutralisation. What is the concentration of NaOH?

10. The Molar Volume of Gases If a substance in the reaction is a gas, then the number of moles can also be calculated from the volume of the gas. Number of moles = Volume of gas (dm3) Molar Gas Volume (dm3)

The molar gas volume depends on the temperature and pressure of the system. At r.t.p. 1 mole of a gas has a volume of 24dm3 Example: What volume of H2 will react with 24dm3 of O2 to form water (at r.t.p.)? i) 2H2(g) + O2(g)  2H2O(l) ii) 2 moles of hydrogen gas react with 1 mole of oxygen gas. iii) If 1 mole is 24dm3 then 2 moles is 48dm3 iv) Therefore 48dm3 of hydrogen are needed.

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Description: This is a seven section guide to help those studying high school level Chemistry. It covers the topics of, -Moles -Moles in Gases -Concentration -Calculations of Reacting Masses -Empirical and Molecular Formula -Solution Calculations -Molar Volumes of Gases