Question

In a gas forming experiment, we did not consider the volume of the reactants occupy in the baggie when we did our calculations for the reactant amounts. Will this oversight cause the pressure inside the bag after the reaction to be higher or lower than the atmospheric pressure in the lab?

Answer 1

In a gas forming experiment, the volume of the reactants inside the baggie was ignored. This is a valid assumption only when the reactants are solids. Solids are known to be compact and occupy little space; hence, we can ignore the volume occupied by the solid reactants in comparison to the volume of the gaseous product generated.

However, if we are absolutely particular about the exact pressure of the gas, we need to consider the volume of the reactants.

It is assumed that since the reactants occupy no volume, the entire volume inside the baggie is occupied by the gas, i.e, the volume of the gas inside the baggie is equal to the volume of the baggie. However, when we consider the volume occupied by the reactants, the volume of the gas will be lower than the volume of the baggie. This is due to the fact that the total space inside the baggie is constant; if the solid reactants take up some of that space, then the space available to the gas will be low. Consequently, the volume of the gas is lower than the reported/accepted value.

If we assume that the gas behaves ideally, we will deduce that the pressure of the gas is inversely proportional to the volume of the gas, i.e, the higher the pressure, the lower the volume and viceversa. Since the volume available to the gas is now low, the pressure of the gas will be higher than the atmospheric pressure in the lab