Acids and bases
Acid Base Titration Using acid-base titration to find mass of oxalic acid
Acid Base Titration
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- I've taken this problem from Chapter 4 of the Chemistry &
- Chemical Reactivity book by Kotz, Treichel and Townsend,
- and I've done it with their permission.
- So let's do this example.
- A 1.034 gram sample of impure oxalic acid is dissolved in
- water and an acid-base indicator added.
- The sample requires 34.47 milliliters of 0.485 molar
- sodium hydroxide to reach the equivalence point.
- What is the mass of oxalic acid, and what is its mass
- percent in the sample?
- So before we even break into the math of this, let's just
- think about what's happening.
- We have some oxalic acid, which looks like this.
- It's really two carbolic acid groups joined together, if
- that means anything to you.
- Watch the organic chemistry play list if you want to learn
- more about that.
- So we have a double bond to one oxygen, and then another
- bond to a hydroxide.
- We have that on the other carbon as well.
- This right here is what oxalic acid is.
- And it's an interesting acid, because it can actually donate
- two protons.
- This proton can be nabbed off, and this proton can also be
- And it's actually resident stabalized.
- If that doesn't mean anything to you, don't worry.
- You'll learn more about that in organic chemistry.
- But the important thing to realize here is that there's
- two protons to nab off of it.
- Now each molecule of sodium hydroxide-- remember when you
- put it in the water it really just dissolves, and you can
- really just think of it as hydroxide-- each molecule of
- hydroxide can nab one of the hydrogen protons.
- So for every one molecule of oxalic acid, you're going to
- need two hydroxides-- one to nab this hydrogen proton, and
- then another one to nab that hydrogen proton.
- So let's write down the balanced equation that we're
- dealing with here.
- So we're going to start off with some oxalic acid.
- So that has two hydrogens-- so it's H2-- two carbons, and
- then four oxygens-- O4.
- It's dissolved in water, so it's an aqueous solution.
- And to that, we're going to add sodium hydroxide.
- Now I just told you that you're going to need two of
- the hydroxides to fully neutralize the oxalic acid.
- So you're going to need two of them.
- And this is also in our aqueous solution.
- And once the reaction happens, this guy will have lost both
- of the hydrogen protons, so let me draw that.
- So it will look like this.
- No more hydrogen, so it's C2O4.
- It'll have a negative 2 charge.
- And actually, you could imagine that it might be
- attracted to these positively charged sodiums. And 2 sodiums
- in particular.
- So this has a negative 2 charge.
- We could even write it there if you want-- 2 minus.
- And then you could have the sodiums over here.
- You have these two sodiums that have two plus.
- And this entire molecules becomes neutral.
- They are attracted to each other.
- They are still in an aqueous solution.
- And then, the hydroxide nabs the protons, and then you are
- left with just water.
- So plus 2 moles-- or 2 molecules depending on how
- we're viewing this-- plus two waters.
- I'll just use that same orange color.
- Plus two H2Os.
- One of the hydrogens in each of the water molecules are
- coming from the oxalic acid, and so two of these hydrogens
- in these two moles of the water are coming from one
- entire molecule of oxalic acid.
- Now let's actually do the math.
- We have 34.47 milliliters of the solution that has the
- sodium hydroxide.
- And I'm just going to convert that to liters just so it's
- easier to deal with the molarity right over there.
- So we have 34.47 milliliters-- we could write it of the
- solution, but we understand that, that's the case.
- So let's just So this is times, we have one liter for
- every 1,000 milliliters.
- And then this will give us-- the milliliters cancel out--
- 34.47 divided by 1,000 is 0.03447 liters of this 0.485
- molar sodium hydroxide solution.
- So let's figure out how many actual molecules of sodium
- hydroxide we have. This is the solution.
- And we know its concentration, 0.485 molar-- so let me do
- that in a different color-- 0.485 molar, this information
- allows us to figure out the actual
- molecules of sodium hydroxide.
- So we want to multiply this by-- we have 0.485 moles of
- sodium hydroxide for every 1 liter of this solution.
- That's what the molarity tells us.
- We have 0.485 moles per liter.
- So the liters cancel out, and then now we're going to
- actually have to get a calculator out.
- And this'll tell us how many moles of sodium hydroxide we
- have in this solution.
- So let me get my calculator.
- There we go.
- All right, let me just multiply these two numbers.
- So we have 0.03447 times 0.485 is equal to-- let me put this
- down here-- 0.167.
- And we only have three significant digits here, so
- we're going to round to three significant digits.
- So we'll just go with 0.0167.
- So let me move that over off the screen.
- So this is going to be equal to 0.0167, and all we have
- left here are moles of sodium hydroxide.
- Now we know that this many moles of sodium hydroxide are
- going to completely react with however many moles of oxalic
- acid we have. Now we know that we need two moles of this for
- every mole of oxalic acid.
- Or for every mole of oxalic acid that completely reacts,
- we need two moles of this.
- So let's write that down.
- And then you color.
- So we need two moles of sodium hydroxide, we got that from
- our balanced equation right there, and it's obvious it
- needs one mole, or one molecule will take this
- proton, and then you need another molecule
- to take that proton.
- So we need two moles of sodium hydroxide for every one mole
- of oxalic acid.
- For every one mole of H2C2O4.
- So essentially, we are just going to divide
- this number by 2.
- Let me get the calculator back.
- So we're just going to divide 0.0167 divided by 2.
- Once again, three significant digits 0.00835.
- So this is going to 0.00835 moles of oxalic acid, H2C2O4.
- So we have the number of moles, but we to
- figure out the mass.
- And we know the molar mass of oxalic acid.
- Let me write these down.
- We know that hydrogen has a molar mass-- let me write it
- this way-- molar mass if you have a mole of hydrogen, it
- has a molar mass of one gram.
- If you have-- and this comes from its atomic weight-- if
- you have carbon its molar mass is 12 grams. And if you have
- oxygen, its molar mass is 16 grams.
- So what's the molar mass of oxalic acid?
- Well we have two hydrogens, so that's going to
- be two grams, right?
- 2 times 1 gram.
- That's the hydrogens there.
- We have two carbons, so it's going to be plus 24 grams. 12
- grams for each of these carbons.
- And then we're going to have four oxygens that weigh in, if
- we have a mole of them, at 16 grams. So that's
- going to be plus 64.
- So what does this come out to?
- 24 plus 64 is 88, right?
- 2 plus 6 is 88, right.
- So it's 88 plus 2 more is 90 grams. So if you had a mole of
- oxalic acid, it would be 90 grams. So we could say 90
- grams per mole of H2C2O4.
- So let's go back to the math here.
- I'll rewrite it over here.
- We know we're dealing with 0.00835 moles of
- oxalic acid, H2C2O4.
- And now we know its molar mass.
- We know that there are 90 grams-- let me do this in a
- different color, this color's getting motonous-- we know
- that there are 90 grams of H2C2O4 for
- every one mole of H2C2O4.
- This is its molar mass.
- And now we just multiply this number, and we'll figure out
- the grams of oxalic acid.
- That and that cancels out.
- Then we just take the number that we had and multiply it by
- 90, so times 90.
- This just says the previous answer, which is the number of
- moles of oxalic acid times its molar mass will tell us the
- grams of oxalic acid.
- So we get 0.75.
- I'll just round it to 2 since we only have
- three significant digits.
- The 90 isn't exact, so it's a little bit-- but I'll just
- round it to three significant digits.
- So 7--.752.
- This is equal to 0.752 grams of oxalic acid: H2C2O4.
- And I think we've answered part of the question.
- So the first question is, what is the mass of oxalic acid?
- We've just answered it.
- That answer right there is 0.752 grams.
- Now the next part is, what is its mass
- percent in the sample?
- Well the sample of impure oxalic acid right over here,
- was 1.034 grams. So we just have to say, what percentage
- is 0.752 of 1.034?
- So let's get the calculator back.
- So we have the 0.752 divided by 1.034 and we get 72.7%.
- So the answer to the second part right
- over there is, 72.7%.
- We were able to figure out that this impure oxalic acid
- sample is 72.7% actual oxalic acid.
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