### Video Transcript

Sulfur burns in oxygen to form sulfur dioxide. What is the mass of sulfur needed to produce four grams of sulfur dioxide?

In this reaction, we have sulfur, symbol S, reacting with oxygen gas, O2, to produce sulfur dioxide, SO2, which is also a gas under standard conditions. The balanced equation is S solid plus O2 gas reacting to form SO2 gas. The reaction is an example of a combination reaction because more than one reactant is combining together to form a new product. We have said that the equation is already balanced. And in this case, the stochiometric coefficients are one is to one is to one. Using the coefficients, we can use the moles of one of the substances to determine the moles of another substance produced or needed.

At this point, we do not have any more values. Let’s see what data we have. We are told that four grams of sulfur dioxide are needed to be produced, so we can write this bit of information under SO2. We are also asked what the mass of sulfur is that is needed to produce this four grams of sulfur dioxide. So under sulfur, we could write mass equals question mark. We are not given any other data, and we cannot directly determine the mass of sulfur needed from the four grams of sulfur dioxide to be produced. We said we can only relate data from two different substances using the coefficients to determine relative number of moles. So our first step is to try determine the number of moles for one of the substances.

If we can work out the molar mass of SO2, we can then use its mass and molar mass together to determine its number of moles. From this, we could then determine the number of moles of sulfur needed according to the mole ratio of the equation. We could then work out the molar mass of sulfur from the periodic table. And using this and the number of moles we just calculated, we can calculate the mass of sulfur needed to produce four grams of sulfur dioxide. So let’s begin our calculations.

The first step, we said, was to determine the molar mass of SO2. From the periodic table, we get 32.06 as the molar mass of sulfur and 15.999 as the molar mass for oxygen. But there are two oxygen atoms, so we multiply 15.999 by two. And we get 64.058 grams per mole. We have not been asked for a specific number of significant figures in this calculation. So for now, let’s keep the three decimal places.

The next step, we said, was to determine the number of moles of SO2. If we take the mass given to us divided by the molar mass we have just calculated, we get a number of moles of sulfur dioxide of 0.062 moles. We know from our balanced equation that one mole of sulfur produces one mole of SO2. Now we can relate the moles of SO2 we have just calculated, 0.062, to determine the moles of sulfur that is needed. And we see that it is the same number of moles. We need 0.062 moles of sulfur to produce 0.062 moles of sulfur dioxide.

Let’s now convert this mole value of sulfur to a mass value. We will use the same equation as earlier, this time rearranged so that mass is the subject of the formula. Let’s clear a bit of space so that we can do this calculation. We can plug the mole value for sulfur we have just calculated in for the number of moles. And we know that the molar mass of sulfur is 32.06 grams per mole. We get a mass of sulfur of 1.98772 grams, and here I’ve chosen two decimal places. If we round off to two decimal places, we get 1.99 grams. Finally, the mass of sulfur needed to produce four grams of sulfur dioxide is 1.99 grams.