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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: ganz on May 23, 2006, 06:40:02 PM

Title: Chemistry problem
Post by: ganz on May 23, 2006, 06:40:02 PM: Hi, Im having trouble trying to solve this problem and im not sure how to start it.

A solution is prepared by dissolving 10.00 grams of a monobasic salt in a total volume of 1.00 litres. The pH of the solution was measured at 13.07 and a conductivity experiment determined that this salt was completely ionized in solution. What is the molecular weight of this monobasic salt?
Title: Re: Chemistry problem
Post by: wereworm73 on May 23, 2006, 07:29:10 PM: Since your monobasic base is 100% ionized, one mole of it will react completely with one mole of H⁺. So, you just need to find out how many moles of H⁺ (from the water) reacted when the base was added. That will give you the number of moles of your base. Once you have that number, you can get grams/mole for the molecular weight.
Title: Re: Chemistry problem
Post by: ganz on May 23, 2006, 08:17:37 PM: Thanks for the help but im not sure how to find either of those could you please show me step by step?

Thank you
Title: Re: Chemistry problem
Post by: mike on May 23, 2006, 08:27:42 PM: pOH = -log₁₀[OH^-]

[OH^-] = ____ mol.L^-1

v = 1.00L

and

m = 10.00g

using:

n = m/M

and

c = n/v

you should be able to work it out..
Title: Re: Chemistry problem
Post by: champ on January 17, 2008, 08:14:12 PM: is the answer for this quesion is 0.77g/mol or somthing else..??? please need help here!!!!!!!!!!!
please correct me thanx .
Title: Re: Chemistry problem
Post by: Borek on January 18, 2008, 03:19:49 AM: Quote from: champ on January 17, 2008, 08:14:12 PM
is the answer for this quesion is 0.77g/mol or somthing else..??? please need help here!!!!!!!!!!!
please correct me thanx .

For sure more than that, lightest possible substance - atomic hydrogen - has molar mass of 1 g/mole. Base must have at least about 23 g/mol (LiOH).
Title: Re: Chemistry problem
Post by: AWK on January 18, 2008, 07:22:51 AM: Quote from: champ on January 17, 2008, 08:14:12 PM
is the answer for this quesion is 0.77g/mol or somthing else..??? please need help here!!!!!!!!!!!
please correct me thanx .
Explore hint of mike.
Calcutate correctly pOH then [OH^-]
Title: Re: Chemistry problem
Post by: champ on January 18, 2008, 11:23:55 AM: what will be the rite answer???? Please help me here??? thanxx
Title: Re: Chemistry problem
Post by: AWK on January 18, 2008, 11:45:13 AM: Between 70 and 100.
Title: Re: Chemistry problem
Post by: DigiTan on January 19, 2008, 03:26:41 AM: I also got a small molecular weight from this and arrived at 1.16 grams/mole. What do you do after you calculate the moles of hydroxide?

[edit]
Okay, disregard that. I just got something in AWK's range. Be sure to double-check your minus signs while dealing with the the LOG and 10^x terms.
Title: Re: Chemistry problem
Post by: champ on January 19, 2008, 10:01:53 AM: Is the Answer for this question "85.11g/mol".?.. thats the rang AWK told me ... if i am wrong then please correct me... it will be highly appreciated.. thank you very much..
Title: Re: Chemistry problem
Post by: Borek on January 19, 2008, 10:59:31 AM: Seems OK.
Title: Re: Chemistry problem
Post by: DigiTan on January 19, 2008, 01:47:42 PM: Same as what I got. What exactly is a monobasic salt? Do they always make total dissociation in water or does that name have some other meaning?
Title: Re: Chemistry problem
Post by: champ on January 20, 2008, 06:57:23 PM: it might help but i am not suhttp://www.freepatentsonline.com/4647392.htmlre..
Title: Re: Chemistry problem
Post by: DigiTan on January 20, 2008, 08:05:55 PM: It didn't really explain what a monobasic salt is, only that one is used. 99% percent was organic chemistry I haven't even gotten to yet.

Does "monobasic" have some relationship with "monoprotic." They sound like opposites, but I hear them described as having the same meaning.
Title: Re: Chemistry problem
Post by: AWK on January 21, 2008, 07:05:14 AM: Monobasic means a salt with one OH group not neutralized, in the case of eg etylene diamine it means H2NCH2CH2NH2.HCl.
Se also (di)basic copper carbonate CuCO3.Cu(OH)2
Title: Re: Chemistry problem
Post by: DigiTan on January 21, 2008, 02:47:19 PM: So even though H2NCH2CH2NH2 doesn't have an oxygen, it could still be a monobasic salt?
Title: Re: Chemistry problem
Post by: AWK on January 22, 2008, 01:10:52 AM: H2NCH2CH2NH2.HCl (monohydrochloride) is a monobasic salt
Title: Re: Chemistry problem
Post by: nerd on January 24, 2008, 01:03:44 PM: 1. pOH=14.00-13.07=0.97
2. [OH-]=10^-0.97=0.12M
3. CxV=n=0.12Mx1.00L=0.12mol
4. B/C 100% ionized n(salt)=n(solution)
5. M=m/n=10.00g/0.12mol=85.11g/mol

For my answer the Molar mass makes sense, but please check if it's correct!
Title: Re: Chemistry problem
Post by: Crazy Wood on January 26, 2008, 07:05:45 PM: Quote from: nerd on January 24, 2008, 01:03:44 PM
1. pOH=14.00-13.07=0.97
2. [OH-]=10^-0.97=0.12M
3. CxV=n=0.12Mx1.00L=0.12mol
4. B/C 100% ionized n(salt)=n(solution)
5. M=m/n=10.00g/0.12mol=85.11g/mol

For my answer the Molar mass makes sense, but please check if it's correct!

I did the same thing but your calculations are kinda wrong i think...

1. pOH = 14.00 - 13.07 = 0.93
2. [OH-] = 10 ^ -0.93 = 0.117 (don't round this... or else you'll get another answer like 83.33 in the end shown in step 4)

3. CxV=n=0.117Mx1.00L=0.117 mol

4. M=m/n=10.00g/0.117mol=85.47g/mol (if you rounded 0.117 to 0.12 you'll probably get something like..83.33)

Tell me if what I did is right.. i need an answer by Wednesday :(
Title: Re: Chemistry problem
Post by: Kryolith on January 26, 2008, 07:15:27 PM: Quote from: Crazy Wood on January 26, 2008, 07:05:45 PM
Tell me if what I did is right.. i need an answer by Wednesday :(

Your calculation is correct.
Title: Re: Chemistry problem
Post by: Borek on January 26, 2008, 07:16:28 PM: Quote from: Crazy Wood on January 26, 2008, 07:05:45 PM
2. [OH-] = 10 ^ -0.93 = 0.117 (don't round this... or else you'll get another answer like 83.33 in the end shown in step 4)

While it doesn't look so, all numbers (ie yours and nerds) are consistent. You have just two sig figs in the input data, so using more in the final answer doesn't make much sense. This means 85 is a reasonable way of writing down the molar mass and both results (85.11 and 85.47; you have used different numbers of guard digits hence the difference) round down to the same number.
Title: Re: Chemistry problem
Post by: Kryolith on January 26, 2008, 07:45:43 PM: Of course you are right Borek. 85.1 g/mol is the correct answer. I just used the value posted by Crazy Wood and did the calculation backwards ::) (shame on me)