So, let's take a look at some questions regarding moles and molarity
The first question asks: two solutions of the same non electrolytic substance are mixed in the following manner
400 milliliters of a 1.5 molar solution plus five hundred and twenty milliliters of a 1.2 molar solution
What is the concentration?
of the resulting solution
Then the second question asks: how many moles of magnesium phosphate will contain
0.25 moles of oxygen atoms? So some different concepts here regarding moles and molarity
So check out some of my previous tutorials if you need some help and when you're ready give this a try
So looking at this first question, we want to find the concentration of the resulting solution and
concentration in molarity we're talking about moles per liter. So we need the total moles and we need the total liters
So if we need the total moles, we need to find the moles in each of the individual solutions
So let's take this first solution. We know that we have 480 milliliters
So let's let's put that as 0.48 liters
And then let's multiply that by 1.5 moles per liter because it's a 1.5 molar solution
so by doing this we are finding out that there are
0.72 moles of this solute in that first solution
Then let's do the same thing for the second solution
We have zero point five 2 liters of that second solution times one point two moles per liter because it is a 1.2
molar solution and so that tells us we have
0.62 moles in that second solution and now to get the
concentration of the total resulting solution we need the total moles over the total liters and the
total moles is just the sum of these - so 0.72 plus
0.62 and then the total leaders. We're just adding these two volumes
So we have zero point four eight liters plus zero point five two liters
And so if we do that arithmetic we get one point three 4 molar as the concentration for the resulting solution
Now for this second question we're talking about
Magnesium phosphate so first we need to know what the molecular formula is for magnesium phosphate
We need to know how many atoms of each element are in this compound
so magnesium phosphate that is definitely going to be an ionic compound because we have a polyatomic ion phosphate in there and
Magnesium ions at the common charge for magnesium is two plus. So we have magnesium two plus ions
And then the phosphate ion, we just have to have this memorized like some of the other polyatomic ions
This is po4 3 - that is what the phosphate ion is
So what is the combination in which magnesium ions and phosphate ions will come together to generate a neutral compound
Well that is going to be mg 3
Po4 2 that way we have a total of 6 + and a total of 6 - so that does add up to neutral compound
So we have 3 magnesium ions and 2 phosphate ions
And so what we need to do now, so now we know exactly what's going on and how many oxygen atoms there are
Per formula unit here. And so we have
0.25 moles of oxygen atoms. And so what we need to do is we need to multiply that by
1 formula unit of magnesium phosphate over 8
oxygen atoms right because each
Phosphate ion has four oxygen atoms and there are two phosphate ions per formula unit
So that is a toxin atoms per formula unit. And that means that
0.03 one moles of magnesium phosphate will contain
0.25 moles of oxygen atoms
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