Finding Moles from ML: Understanding Substance Volume

[Euphoriet]

I'm sorry but if I have a substance.. and I know it's made up of two specific elements.. and all I have is the quantity in ML, how can I find the amount of moles for it?


Another question.. heh.. if I have a measurement of ML.. does that mean.. that's the substance volume?


Thanks

 

[dextercioby]

U need the molar concentration (a.k.a.the molarity).And,if that ML stands for milliliters,then it should be spelled ml...

And yes,a milliliter is a unit for volume of a fluid.

Daniel.

 

[Euphoriet]

heh sorry about the ml... ok the substance is HCl.. is it possible to find the "molarity" of it?

 

[dextercioby]

U needn't.U need to peform a rection involving a known quantity of reactant and HCl.Then u'll know the mass of HCl in the solution.Divide the mass by 36.5g/mol and u'll find the # of moles in the initial solution (assuming the reaction is stoichiometric).

Daniel.

 

[whozum]

Molarity = $$\frac{Moles of substance}{Volume of solution}$$

Moles of substance = $$Molarity * Volume$$

 

[Euphoriet]

Oh man... I don't have this information.. so I'm guessing I'm pretty much stuck..

This pretty much sucks, it was a lab using phenolphthalein to neutralize the HCI and an antacid. Then we used NaOH to titrate it.

The problem is all the information gather was pretty much the:

1. HCL used in ml
2. amount of phenolphthalein
3. weight of antacids
4. amount of NaOH to titrate

So.. with all this being the information I have.. how would I even consider finding the molarity of HCI.

 

[dextercioby]

Well,write the equations of the reactions taking place...It's the first step to do.

Daniel.

 

[Euphoriet]

I'm sorry I'm really bad with chem..

mind pointing out how it would look.. or giving me a sample "problem"..?

Like..

1. how would I set up the equations.. when I'm adding two substances... , how would I take into accound measurements in ml
2. How would this information help me?
3. Since I'm adding more to that mixture.. wouldn't it get pretty complicated after the first try?
4. I really don't see how molarity has any significance after the first step.. which is to add HCl and phenolphthalein?

 

[dextercioby]

The simplest would be

$$HCl+NaOH\rightarrow ...?$$

Assume that u know the mass of NaOH and u want to determine the mass of HCl...Assume stoichiometry.

Daniel.

 

[Euphoriet]

Thanks.. hmm but no I do not know the mas of any of these substances other then the mass of the antacid (tablet dissolved in the HCI , this was basically the first step, it was boiled for 1-2 mins).

One thing.. even if I did have the mass of one thing.. wouldn't I still need the resulting mass (2 numbers in an equation of 3) to figure out HCI?

 

[pack_rat2]

...t was a lab using phenolphthalein to neutralize the HCI and an antacid. Then we used NaOH to titrate it...

The phenolphthalein is an indicator: it changes color at a particular (narrow) pH range, and thus tells you when the HCl has been neutralized. It doesn't neutralize anything itself, and the amount used, which should be VERY little, is generally unimportant. The usual procedure for titrations is as follows... For HCl, it is first standardized. Hydrochloric acid consists of a solution of hydrogen chloride in water. Its concentration can vary a little over time, so the lab acid can not be obtained as a standard in its own right, off the shelf. You will normally make an approximate solution of say, ~ 0.1M, and then titrate that with a basic solution of accurately known concentration. Na2CO3 is commonly used for this. First, it's thoroughly dried. Then a quantity is weighed on an analytical balance. That's put in a volumetric flask. Water is added, to dissolve it, then it's filled with more water, up to the mark on the flask neck. NaOH isn't a good choice, since it's too hygroscopic (it can absorb moisture while you're trying to weigh it). But if you DID use NaOH for this purpose, you would have weighed it first, then made a solution with it of known concentration. Is this what you did?

 

[pack_rat2]

...I do not know the mas of any of these substances other then the mass of the antacid...

OK, I should have read this post first! Apparently, you used the antacid to standardize the HCl. What chemical is it? (Probably CaCO3, but could be Mg(OH)2, NaAlO2, maybe something else...)

 

[Euphoriet]

lol.. sorry its actually alka seltzer and tums =-/.. heh.. its a very .. "domestic" lab... and the labels on these thigns are not very helpful at all.

The NaOH was already in liquid form.. it was just added and measured in ml.. the antacid were two different 'products'... weighted first.

 

[Euphoriet]

Hm.. I can't find which antacid (tums or alka s.) provides the better ratio of moles of acid neutralized per grams of the antacid.

Does anyone know if they provide any type of information .. something like it somewhere online?