Sciencemadness Discussion Board - I Can't Do The Maths Myself :(

I Can't Do The Maths Myself :(

K12Chemistry - 20-2-2013 at 08:32

If I had 100g of cuso4 and dissolved it then electrolysed it, how much sulfuric will I get. how much 98% sulfuric acid?

does it depend on how much water?

Hexavalent - 20-2-2013 at 09:57

Quote: Originally posted by K12Chemistry

If I had 100g of cuso4 and dissolved it then electrolysed it, how much sulfuric will I get. how much 98% sulfuric acid?

does it depend on how much water?

What's "cuso4"?
Do you mean "sulfuric acid"?

Yes, the final concentration of acid you will get depends on the amount of water present. It is highly unlikely that you will achieve 98% concentration with this method and considering what materials you'll likely have available. Boiling down the dilute acid is an option, but for someone who cannot do this maths, I would not recommend it.

K12Chemistry - 20-2-2013 at 10:17

No I mean that if I got 500ml of 49% sulfuric acid, I need you to tell me how much 98% I would get (250ml). I know that it is basically impossible to get 98% with this method.

I have boiled it down before many times, so I do have experience.

Anyway so how much?

[Edited on 20-2-2013 by K12Chemistry]

woelen - 20-2-2013 at 10:17

Don't write cuso4, please be precise with symbols.

The compound is CuSO4. And I'm quite sure that your compound even is CuSO4.5H2O (the blue salt, hydrated copper sulfate). Anhydrous CuSO4 is a white compound.

Now the math. First you have to compute the molar mass of CuSO4.5H2O. This can be done by looking up the molar mass of copper, sulphur, oxygen and hydrogen. The compound has 1 atom of copper, one atom of sulphur, 9 atoms of oxygen and 10 atoms of hydrogen. Do you understand how I derived these values? From these compound the total molar mass.

If you have 100 grams, then the number of moles of copper sulfate pentahydrate is 100 / M, where M is the number you computed.

The next step is to compute how much sulphuric acid you can obtain. What is the net reaction of electrolysing a solution of copper sulfate? Try to write down that reaction and determine how many moles of H2SO4 you can get for each mole of CuSO4.5H2O. You know how much copper sulfate you have, then you can compute how many moles of H2SO4 you get. From that you can compute the number of grams of H2SO4.

So, now it is your turn again. Do the math, according to what I explained. If you get stuck somewhere feel free to post again, otherwise please post the results of your computation.

We are not going to spoonfeed you the answers, we want you to understand things, such that you can do the math yourself in the future and can do more interesting and better experimenting in the future.

K12Chemistry - 20-2-2013 at 10:18

If I start with 100g of copper sulfate then do I get ~100ml of 98% sulfuric acid (stoichometrically).

elementcollector1 - 20-2-2013 at 10:20

This is a simple molar mass calculation.
How many moles of CuSO4 do you have?

According to a balanced equation, if everything reacts, how many moles of H2SO4 will this make?

How many grams of H2SO4 is this?

Given the lowest amount of water that 100g CuSO4 will dissolve in (from solubility data @ 20 degrees C), and the grams of sulfuric acid made, what will be your end concentration in % by mass?

If you can figure out these questions (high school level chem problems), then you can work out the answer to your question.

K12Chemistry - 20-2-2013 at 10:20

thanks woelen, I'll try later I have homework 2 do

Hexavalent - 20-2-2013 at 10:24

Quote: Originally posted by K12Chemistry

thanks woelen, I'll try later I have homework 2 do

You will be taken much more seriously as an experimenter and an individual if you write words out properly, i.e. "to do" as opposed to "2 do".

Finally, just a note about calculating formula masses as woelen mentioned: you find Ar (relative atomic masses) on the periodic table - usually the larger number seen - or, for larger, more complicated compounds, you can often look them up in a handbook or online. This is particularly useful if you have a large organic compound and it is easy to miss a hydrogen atom, for example.

elementcollector1 - 20-2-2013 at 10:30

A quick note: Hydration can be very important when calculating molar masses. I've messed up more than one calculation because I forgot to factor in those water molecules.

Magpie - 20-2-2013 at 10:42

I suggest you start every chemistry problem by writing the balanced chemical equation. It not only shows the chemical change but is a material balance. Misconceptions you may have often show up when you try to write the balanced chemical equation.

And yes, please try to write in a formal manner. Science requires rigor! I know it must be hard for today's texting/tweetering iphone generation.

Draconic Acid:

As you say "filtrate" is not a verb. But what is "filterate"? I've seen that used also on this board.

[Edited on 20-2-2013 by Magpie]

DraconicAcid - 20-2-2013 at 10:51

Quote: Originally posted by Magpie

Draconic Acid:

As you say "filtrate" is not a verb. But what is "filterate"? I've seen that used also on this board.

[Edited on 20-2-2013 by Magpie]

The filtrate is the solution that goes through the filter. "Filterate" is a spelling error. I once had a class of students where at least half of them insisted on writing "The solution was filtrated", which drove me crazy (actually, I just checked my Oxford Concise dictionary, and it tells me that it's acceptable! AAUUGH!). That, and "glasswear", which sounds really uncomfortable.

Spelling and grammatical errors don't bother me as much on a board like this, because I know that not all chemists are anglophones. But I know my students are. Supposedly.

zed - 20-2-2013 at 18:27

"If I start with 100g of copper sulfate then do I get ~100ml of 98% sulfuric acid (stoichometrically)."

At a glance, you should know, the answer is NO! 98% Sulfuric Acid is quite dense.
Almost twice as dense as water. As such, even at a 100% yield, and even if your 100 Grams of copper sulfate were completely anhydrous, it could not produce 100 ml of H2SO4 (184grams).

100 Grams of Copper Sulfate cannot possibly produce 184 grams of H2SO4

vmelkon - 23-2-2013 at 07:25

Once you plate out the copper, you are left with H2SO4. I'll let you do the calculations to find the concentration.
After that, you have 2 options. Boil it to drive off the water and when it starts emitting white fumes, the conc is in the 90% (I think).

Option 2 is to do electrolysis with platinum electrodes. Eventually, you can reach 95%. I had a paper that described this. It requires both electrodes to be close and stirring the solution to form a certain anion (S2O8(2-) I think). At high concentration, a few milligrams of sulfur start to deposit on the electrodes.