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Taximes

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Unearthly Stew

Basically don't you just need to convert 10.12 mL into grams, then subtract the Molar Mass of HCl?

Edit: I have a pretty good feeling this is wrong.

Unearthly Stew

Also, I think your equation is off, shouldn't it be Mg(s) + 2HCl(aq) -->MgCl2(aq) + H2(g) ?

If you're working with stoichiometry the 2 moles of HCl will have a pretty big impact.

HCl is approx 36.45 g/mol if that helps any...

I'm too tired to complete the math.

Taximes

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Oats

There are places you can look for such information. Just be sure to cite your sources!

Cowboy Bebop

Is this not just a simple case of using the formula M=CV?

It's been a while since I did chemistry.

Taximes

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Palmer Eldritch

I'm pretty sure you need the volume of MgCl29(aq), since you don't have its concentration.

Fuzzy Cumulonimbus Cloud

Cowboy Bebop wrote: »

Is this not just a simple case of using the formula M=CV?

It's been a while since I did chemistry.

There are a few equations, try each one.
I think M=CV is what you are looking for here though.
Also, V1C1= V2C2
Also, volume needed * g/mol * mol/L = grams of whatever you need to add to make that solution.

Demerdar

you probably shoulda boiled the water off at the end of the experiment and then weighed the remaining material. unless you cant.

Plutonium

Taximes wrote: »

tl;dr - How much water is in 10.12mL of 1M Hydrochloric Acid?

Disclaimer:This is what you asked for, but I'm not sure it's exactly what you'll need, but whatever, it's what occurs to me at this time at night.

First of all, because it's in solution in water, you can assume the density if 1g/ml

So that means that your 10.12mL Solution weighs 10.12 grams.

Secondly, because you have the molarity and volume, you can calculate how many Mols of HCl are in solution.

10.12mL HCL x 1 L / 1000mL x 1 Mol / 1 L = 1.012 x 10^-2 mols HCl

Then, because you have the molecular weight of HCl (36.46g/mol), you can calculate the mass of HCl dissociated in the solution.

1.012 x 10^-2 mol HCL x 36.46g/1 mol = 0.369g HCL

Now you have the mass of the HCL, so subtract it from the total mass to find the mass of water.

10.12g - 0.369g = 9.715g H2O, which comes out to be 0.541 mols of H20

Walter

Ochem TA here
Plutonium's method to find the mass of water isn't really something I've ever seen but I don't see any reason it shouldn't work.

SanderJK

You have 1M of HCl, of which you added 10.12ml. This means in total you added 10.12mM of HCl. 2 equivalents of HCl react with 1 equivalent of Mg (to form MgCl2), so you had 10.12/2=5.06mM of Mg in the beaker. Look up the molecular weight of Mg, multiply it by the amounts of moles, and you get your starting weight.

Regrettable

You can work out how many mols of HCl and thus chlorine you had from the amount reacted and the concentration, and half as many mols of magnesium will have reacted as chlorine (Mg+ + 2CL- -> MgCl2) , so just halve that number and multiply by their respective molar masses to get your masses of chlorine and magnesium.
Solubility and mass of water don't really have anything to do with your calculations as long as you know the concentration of the HCl.

Walter

SanderJK wrote: »

You have 1M of HCl, of which you added 10.12ml. This means in total you added 10.12mM of HCl. 2 equivalents of HCl react with 1 equivalent of Mg (to form MgCl2), so you had 10.12/2=5.06mM of Mg in the beaker. Look up the molecular weight of Mg, multiply it by the amounts of moles, and you get your starting weight.

Right, but for some reason he wants the exact amount of MgCl2. He wants to compare the amount that reacted with the HCl to the amount of mgCl2 generated.

OP: Are you SURE this is what you are supposed to be doing? Usually labs like this have you weigh the amount of Mg you started with and calculate how much HCl it should take to completely react. Then you compare how much HCl reacted to what you're calculations are.

Plutonium

That's what I thought also, I'm not sure what exactly he needs, so I just told him exactly what he asked for in the Tl;Dr.

SanderJK

On further review, your numbers are suspicious. You added 10.12ml of solution to the solid, and ended up with exactly 10.12g of solution?

Looking at some data:
http://en.wikipedia.org/wiki/Hydrochloric_acid

The table indicates that the density of HCl(aq) is above 1. (The lowest figure they give is 2.3M at 1.048, so 1M should be near 1.02M roughly). This where the above indicates immediate error. No matter what, even without the Mg added the weight of 10.12 of solution is supposed to be more then 10.12g. I came to the same conclusion another way too, trying to work with the density of water, assuming added the salt to it didn't appreciately change it's volume. If you try to work this way, you end up with a negative amount of Mg(s).

If you really want the bolded question answered, plot a graph from the data in the link above. But most likely you were supposed to weigh the Mg(s). If you have weighed the whole solution in the beaker, you could go back to the lab, use the same buret with the same solution still in it, let it run of 10.12ml again and weigh that. Substract the 2 values, and you should end up with the answer too. Since we're looking for about 0.5g, it should be weighable. But the 10.12g value is almost certainly false, or the 10.12ml. i don't see any way they can both be right.

EDIT: A few minutes of thought, this was an open beaker, in at room temperature? Were you using a fume hood? How long did you take between titration and weighing? Because under a fume hood, any vapor pressure above the liquid will be removed slowly by the airflow, and evaporate the water over time. It won't be much with water, but if those 10.12ml and 10.12g both are measured, it may be a possibility (I still wouldn't count on it).

corcorigan

A sensible person would have weighed the solution in the beaker before as well as after perhaps?

I spent most of chemistry breaking glassware and showering NaOH over my classmates though, so that might not be it.

Dropping Loads

At this point, it appears you didn't collect all the data you needed to precisely determine the answer, so instead you're stuck with a couple of assumptions. The most important assumption is, "Which method of calculating the answer is going to earn me the most points on this lab report?" I'd recommend against Plutonium's answer because although the error is likely to be low in this instance, you would be starting with an assumption which is fundamentally wrong (that the density is 1g/mL). Of course, this is dependent upon your professor and the TA.

The most legitimate answer is to determine the mass of the 10.12mL of 1M HCl solution by looking it up in a table (as proposed by SanderJK), and just admit the errors in your lab report. When I taught O Chem that was always the best way to impress me. It shows you're actually thinking about the problem, which you are.

ALSO: Don't forget to account for the mass evolved as H2 gas. It's an important part of your calculation, although I don't think it accounts for the total error in this case (without doing the math myself).