KEY2CHEM

Law of Conservation of Mass

The law of conservation of mass is another way of stating Dalton’s atomic theory postulate that atoms can be neither created nor destroyed. Atoms are conserved during chemical and physical processes, which allowed for the balancing of chemical equations, as well as calculation of reactant and product quantities. Even during more complex process, such as the global nitrogen or carbon cycles, atoms are still conserved (neither created nor destroyed).

Example 1.

If a $$20\text{ g}$$ ice cube melts, how many grams of liquid water will be formed?

A. $$20\text{ g}$$

B. $$10\text{ g}$$

C. $$5\text{ g}$$

Solution

A. $$20\text{ g}$$

The mass of the starting materials (in this case, the ice cube) is equal to the mass of the end products (in this case, liquid water). Based on the law of conservation of mass, atoms can be neither created nor destroyed.

Example 2.

Which diagram violates the law of conservation of mass?

A.

B.

C.

Solution

A.

The law of conservation of mass states that atoms can be neither created nor destroyed. This diagram violates that principle, as the number of darkened circles differs on the two sides of the reaction (is not conserved).

Example 3.

If $$32\text{ g}$$ of reactant A and $$56\text{ g}$$ of reactant B are combined to form product C as the only product, how many grams of C can be formed?

A. $$32\text{ g}$$

B. $$56\text{ g}$$

C. $$88\text{ g}$$

Solution

C. $$88\text{ g}$$

The law of conservation of mass states that matter is not created or destroyed, so the mass of reactants must equal the mass of products.  $$32\text{ g} + 56\text{ g} = 88\text{ g}$$