Conservation of Mass

In any chemical change, mass is conserved. This simply means that the total mass of the substances you begin with must equal the total mass of the substances you end up with.

In other words, in ordinary chemical reactions, mass is neither created nor destroyed. This is not true in nuclear reactions, where mass can be converted into energy and energy can be converted into mass. But in ordinary experience, mass is conserved. Early measurements confirmed that where mass appeared to have been lost, in fact the missing mass was in the form of a gas, and so escaped from the reaction vessel. When such reactions were carried out in closed vessels, no mass was lost. This came to be known as the law of conservation of mass. Dalton's atomic theory addressed this when he said that chemical reactions involve the rearrangement of atoms and atoms are neither created nor destroyed in chemical reactions.

For example, when 10 grams of calcium are burned in oxygen, 14 grams of calcium oxide are formed. Conservation of mass says that the sum of the masses of the reactants (the substances we start with) must be the same as the sum of the masses of the products (the substances we produce).

10 g + X = 14 g

X = 14 g - 10 g

X = 4 g

This means that 4 g of oxygen must have been involved in our reaction.

Let's consider a slightly more complex example. When 33 g of wood was burned in air, it combined with 32 g of oxygen to form 3 g of ash and a gas mixture of carbon dioxide and water. When the gas mixture was cooled, 18 g of water condensed. How much carbon dioxide was formed?

As in the previous problem, it's helpful to set up an equation to focus on the information we know and how it relates to our problem.

33 g + 32 g = 3 g + X + 18 g

X = 33 g + 32 g - (3 g + 18 g)

X = 44 g

The mass of the carbon dioxide produced in this reaction was 44 g.

This type of reasoning can be applied to any chemical reaction in which all the components (going both in and out) are accounted for. You must know the masses of all the components but one, which will allow you to solve for that mass.