How many moles of iron ions are found in 1.5 liters of 0.50-molar iron three sulfate? A) 0.25 moles, B) 0.50 moles, C) 0.75 moles, D) 1.5 moles, or E) 6.0 moles.
Iron three sulfate is an example of an ionic compound. The three in Roman numerals indicates a charge of three plus. So we know that iron three sulfate contains the iron three plus ion. And the symbol for the sulfate anion is SO₄²⁻. Ionic compounds are neutral overall. And we can use that to work out the formula for iron three sulfate.
If we have just one iron three ion and one sulfate ion, we have an overall charge of plus. But if we have two iron three ions and three sulfate ions, their charges balance out. You can see that we have Fe₂ and SO₄²⁻ and Fe³⁺ and SO₄ three times. So you can use this trick to figure out how many of each you need.
So why did I spend all this time figuring out the formula for iron three sulfate? The question asks for the moles of iron ions. And we need to know how many iron three ions are in iron sulfate before we can do anything else.
Our next step is to look at the solution. We have 1.5 liters of 0.50-molar iron three sulfate. The units molar are equivalent to moles per liter. So we have all the information we need to figure out the moles of iron ions found in 1.5 liters of 0.5-molar iron three sulfate. But before we do that, we need to work out the amount of iron three sulfate in our solution. To do that, we take the volume in liters and multiply it by the concentration. So it’s 1.5 liters multiplied by 0.5 moles per liter, which gives us 0.75 moles of iron three sulfate.
And then we take the amount of iron three sulfate and multiply it by two moles of Fe³⁺ per mole of iron three sulfate. This is because, for each unit of iron three sulfate, we get two iron three ions. And this evaluates to 1.5 moles of iron ions.
So for this question, it was very important that we figured out a formula for iron three sulfate first. Only then could we know the ratio between moles of iron three sulfate and moles of iron ions. And demonstrate that there are 1.5 moles of iron ions in 1.5 liters of 0.50-molar iron three sulfate.