![]() Iodine is a trace element that is naturally present in some foods, is added to some types of salt, and is available as a dietary supplement. For a general overview of Iodine, see our consumer fact sheet on Iodine. For example, if I have 5 g of iron which reacts with the oxygen in the air, the rust (iron oxide) that is formed will have a larger mass than the 5 g of iron that we started with.This is a fact sheet intended for health professionals. This time, the reactant will be a gas whose mass hasn’t been accounted for. This would result in the calcium carbonate appearing to lose mass as the reaction progresses.Īlternatively, other reactions may appear to involve an increase in mass. This can be explained by the fact that one of the products (carbon dioxide) is a gas and has escaped from the flask. ![]() However, if we were conducting this experiment in the lab with the calcium carbonate in a flask on a weighing balance, it may appear that there has been a change in mass during the reaction. If I heat 20 g of calcium carbonate, this will decompose into calcium oxide and carbon dioxide with a total weight of 20 g. Let’s think back to the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide. Since we also have the same number of moles of products and reactants, this means that the mass of the products will always be equal to the mass of the reactants. We’ve learnt that in a balanced equation, the formula masses (Mr) of the reactants will always be equal to the formula masses of the products. This means that the remaining 73% will come from the two atoms of oxygen. This means that 27% of the mass of the whole molecule comes from carbon. We then multiply by 100 to convert it into a percentage. All we need to do is divide the atomic mass of carbon by the formula mass of carbon dioxide. For example, let’s say we need to find out the percentage by mass of carbon in carbon dioxide. You may be asked to calculate the percentage by mass in a compound – this indicates how much weight a particular element is contributing to the whole molecule. Total formula mass of products = 56 + 44 = 100 This is shown below:įormula mass of calcium oxide = 40 + 16 = 56įormula mass of carbon dioxide = 12 + (16 x 2) = 44 Since we know that the Mr of the reactants is equal to the Mr of the products, this means that the formula masses of calcium oxide and carbon dioxide will add to 100. In this reaction, calcium carbonate is the only reactant and we have already calculated its Mr as 100. ![]() You can also use the crossing-over method to work out the formula of a molecule if you know the charges of the ions. Remember that when we are writing the formula of a molecule, we multiply atoms by writing numbers after the atom we want to multiply in subscript. For the whole molecule to be unbalanced, we need two iodide ions to balance out the +2 charge on the magnesium ion. Iodide ions have a +1 charge, since iodine needs to gain 1 more electron to get a full outer shell. ![]() Magnesium forms an ion with a +2 charge, since it needs to lose 2 outer electrons to get a full outer shell. Instead, we have to pay attention to the charges on the ions formed by these atoms. When these elements come together to form a molecule, it may not be the case that one magnesium atom bonds with only one iodine atom. This can be done by looking up their symbols on the periodic table: Mg for magnesium and I for iodine. To start with, we need to figure out the formula of magnesium iodide.
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