Determine Mass of Black Powder Needed to Fire Cannonball

Let's take a look at this mole concept question.

mass of black powder question

We are required to calculate the mass of black powder needed to expel the cannonball.

mass of black powder mole ratio

6 moles of reactants will give 4 moles of gaseous products.

Hence by comparing mole ratio, we can calculate moles of reactants to be 3.00 mol.

Next we need to split these 3.00 mol of reactants to the individual parts.

mass of black powder moles of reactant

Out of 6 parts in total, there are:

- 2 parts KNO3 hence moles of KNO3 is 1.00 mol
- 1 part S hence moles of S is 0.500 mol
- 3 parts C hence moles of C is 1.50 mol

We can then find the mass of each reactant and combine them to find the mass of black powder required to be 135.2g.

mass of black powder mass

Hence the answer to this question will be option B.

mass of black powder answer

There is an alternative way to compare the mole ratio.

We can treat 2 moles of KNO3, 1 mole of S and 3 moles of C to be one unit of black powder.

This means the mole ratio of black powder to gaseous products is now 1 to 4.

mass of black powder alternate mole ratio

Therefore moles of black powder will be 0.500 mol.

We can then find the molar mass and hence the mass of black powder which will be the same value at 135.2g.

mass of black powder mass black powder

Topic: Mole Concept, Physical Chemistry, A Level Chemistry, Singapore

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