A supplied reaction mass ratio in a closed model
A simulation studies a reaction in which magnesium and oxygen form only magnesium oxide. For this exercise the reacting mass ratio is supplied as3parts magnesium to2parts oxygen. ContainerA initially holds1.2grams magnesium and1.6grams oxygen. ContainerB initially holds1.2grams magnesium and0.6grams oxygen. Both containers are closed, neither leaks, no other substance reacts and the reaction proceeds until the limiting reactant is exhausted. These assumptions describe an ideal data exercise, not instructions for carrying out a combustion experiment. A student thinks that every initial gram must become oxide and another thinks that an excess reactant disappears. Track the oxide and any unreacted material separately before calculating a total mass. Use only the supplied ratio and do not infer a different product.
1. Find the product and leftover material in containerA.
Read the worked answer
The1.2g magnesium requires1.2×2/3=0.8g oxygen. Magnesium is limiting. The oxide mass is1.2+0.8=2.0g and0.8g oxygen remains. The total is2.8g, equal to the initial total.
2. Repeat for containerB.
Read the worked answer
The0.6g oxygen requires0.6×3/2=0.9g magnesium. Oxygen is limiting. Oxide mass is1.5g and magnesium left is0.3g, totaling1.8g as initially.
3. Could a reacting open sample gain mass without violating mass conservation?
Read the worked answer
Yes. Oxygen or another specified reactant entering from the surroundings can increase the measured sample mass. Include that entering matter in the system accounting; mass has not been created.