HELP with unknown Monobasic HA question

1. Oct 8, 2005

laker_gurl3

HELP with unknown Monobasic HA question!!

An unknown monobasic acid HA reacts with sodium hydroxide as follows:-

HA + NaOH --->NaA + H2O

Briefly describe a method for determining the molecular weight of the unknown monobasic acid.

Hint - Remember C the concentration of a solution in gdm^-3 and M the molarity (molar concentration) of a solution in moles dm^-3 can be interconverted using the equation:

M = C/(MW)

Where MW is the molecular weight of the substance.

2. Oct 8, 2005

laker_gurl3

anyone have any ideas...? thanks guys

3. Oct 8, 2005

P-man

I would suggest figuring out what the uknown acid is.

4. Oct 9, 2005

laker_gurl3

I was thinking along the line of A could be anything, so let's say A = NO3, therefore NaA would be NaNO3, devise an experiment to find the weight of NaNO3 and break it up to find the weight of each specific part, (Na and NO3) and you'll know the monobasic acid..? I'm not really sure if that makes sense because i dont even know how to explain and devise an experiment to find the weight of a substance like NaNO3..any suggestions or help would be appreciated because i also have no idea how detailed i should go...like to the point of making up an actual experiment?

5. Oct 9, 2005

Gokul43201

Staff Emeritus
Is the concentration of HA known ? If not can you find it out by titrating against NaOH ?

Knowing the concentration of HA, you should be able to set up an equal number of moles each of NaOH and HA. React these with each other and (assuming the reaction goes essentially to completion), you can calculate the mol wt of HA by weighing the products.

Give it some thought and see if you can come up with the right equations to solve for the mol wt.

6. Oct 9, 2005

laker_gurl3

To look for molecular weight, concentration is needed.
Because the concentration of HA is unknown, figuring out the concentration by using the titration method against the basic NaOH is needed. When using the titration method, we can then find the titration point of the unknown acid(HA) which is the titre. From there, go on to find the concentration of the unknown acid by C= n/v (c= concentration, n=moles, v=volume).

how does that sound?

7. Oct 9, 2005

Gokul43201

Staff Emeritus
True, but you must also explain hoow you can find the MW from the conc.

Getting there (on this half of it)...but what exactly is n ?

When you describe what C, n, v are, you need to be more specific.

8. Oct 9, 2005

laker_gurl3

ok how about this, i know what the M.W. of NaOH and say that thhere is about 1 mole of it, the volume, lets say 25ml. i will begin titrating the unknown HA until i find the titration point/ equvalance point. can i find the M.W. of it now?

9. Oct 9, 2005

Gokul43201

Staff Emeritus
What I meant by my previous post was that you need to say that :

C = conc. of HA, v = vol. of HA, n = moles of NaOH consumed in the titration = C(NaOH)*V(NaOH)

Now using C1V1 = C2V2, the conc. of HA can be found. That's one part of it.

Now from the conc of HA, you need to determine its MW. Knowing the concentration, let's say you react 1 mole of HA with 1 mole of NaOH, giving 1 mole of NaA and 1 mole of water. Now if you boil off the water and weigh the product, what do you have ?

10. Oct 9, 2005

laker_gurl3

ok here is my answer so far:

HA + NaOH ---> NaA + H20

ok since HA is monobasic then the moles of NaOH will equal the moles of HA, For example lets say that there is .2 M and 30ml of NaOH, Find the moles of NaOH using C=N/V

So... NaOH moles = HA moles.

Now lets say that there is 25ml of HA being titrated with NaOH, so now it would be 6 mMoles of HA, also lets say there is a blank amount of HA being diluted by 100ml of water or anything doesnt really matter, Now after figuring out the new conc .24 M Now the moles of HA in that 100ml would be .024 moles but now how would i get M.W. i dont have the grams of HA at all.

I understand the question said to describe a METHOD, but maybe an example would help describe what i'm saying much easier..?

Last edited: Oct 9, 2005
11. Oct 9, 2005

Gokul43201

Staff Emeritus
Take 100 mL of HA and weigh it. Now take 100 mL of water and weigh it. What does the difference in weights give you ?

12. Oct 9, 2005

laker_gurl3

so i just weigh the difference in water and HA and that would get me molecular weight?

13. Oct 9, 2005

GCT

Mercy. Isn't all of this written in your text (or lab text)?

One usually obtains the weight of the compound through an analytical balance (.1 mg uncertainty). Then titrate the unknown acid with a standardized solution of sodium hydroxide.

You now have the moles and the corresponding gram value.

14. Oct 9, 2005

laker_gurl3

im telling you everything i know right now, theres nothing else on my lab except what i wrote on the above post. i still do not understand what you are saying GCT, can you further explain yourself?

thanks

15. Oct 9, 2005

Gokul43201

Staff Emeritus
No, that will give you the grams of HA, from which you can find the MW.

GCT : I don't think this is a standard lab. Looks more like a "design your own experiment" kind of thing.

16. Oct 9, 2005

laker_gurl3

Thanks so much!and you are right, its a make your own lab and its a pain.

17. Oct 9, 2005

P-man

How do you find the MW from the grams of HA?

18. Oct 9, 2005

GCT

well at times you have an compound, which may be an acid (although some acids decompose pretty easily when dried)....you weight it with an analytical balance to determine the weight. Then you titrate it with a standard solution of NaOH for instance, that way you can determine the actual corresponding moles of the acid.

So now you have the moles of the acid and its related weight. Molecular weight is termed to be the grams/moles, so simply divide the gram value by the mole value.

Perhaps, but I'm sure the teacher has given enough information though , it doesn't seem to be from a higher level course.

19. Oct 10, 2005

P-man

I get it now.