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April 28, 2015

April 28, 2015

Posted by **Kyle** on Monday, February 16, 2009 at 6:48pm.

How many milliliters of this solution should be diluted with water to produce 600 ml of a solution pH= 11.65?

- Chem -
**DrBob222**, Monday, February 16, 2009 at 8:00pmFirst you need to determine the concentration of NH3 in the final solution.

NH3 + HOH ==> NH4^+ + OH^-

Kb = (NH4^+)(OH^-)/(NH3)

You will need to look up Kb, if it isn't given in the problem, then plug in for OH^- and NH4^+ the OH^- you obtain from a pH of 11.65. That means a pOH of 2.35 and a (OH^-) of about 0.00447 but you need to go through it exactly since I've estimated. (Also, you must determine if the NH3 to use is NH3 - OH or just NH3. The size of the numbers will help you determine that.

Second, you need to determine the molarity of the NH3 solution you have (that's the 6.8% solution).

It has a density of 0.97 g/mL; therefore, 1000 mL will have a mass of

0.97 g/mL x 1000 mL = 970 grams.

How much of that is NH3. That will be

970 grams x 0.068 = 66 g NH3.

How many moles is that. grams/molar mass = moles = 66/17 = approximately 4 M.

Now use M(soln 1) x L(soln 1) = M(soln 2) x L(soln 2).

Post your work if you get stuck. And note that I've estimated at each step along the say so you need to redo all of this but use the exactly values.

- Chem -
**ur class fella**, Tuesday, February 17, 2009 at 12:31amOk here is my solution to it but i am constantly told that it is wrong.

d=0.97g/mL

Mass %= 6.8%

1000mL= 1L

d=m/v m=d X v

m= 0.97g/mL X 1000mL

m=970g

mNH3 = 970g X 0.068=65.96g of NH3

MM of NH3 = 14g/mol + 3 X 1g/mol=17g/mol

nNH3 = 65.96g/17g/mol

n=3.88mol

[NH3]= 3.88mol/L or M

pH=11.65 thus pOH=2.35 thus [OH-]=4.466X10^-3

V=0.625L

Now using the ICE table for NH3 + H2O =(equilibrium sign) NH4^+ + OH^-

Kb=1.80 X 10^-5

I 3.88 0 0

C -x +x +x

E 3.88-x x x

Kb= [NH4^+][OH^-]/[NH3]

1.80 X 10^-5= x^2/3.88-x

neglecting the x because Kb is very small( I hope i am right)

1.80 X 10^-5 X 3.88 = x^2

using calculator and solving I got

x=8.3570X10^-3

similary X is [OH-]= 8.3570X10^-3

Using c1v1=c2v2

v1=c2v2/c1

v1=4.466X10-3 X 0.625/8.3570X10^-3

v1= 0.334L

which is 334mL and is the final answer.

But why in the NAME OF ARHENIUS, is the my answer not right.

DR. BOBB or any chem genius could tell me if I am missing anything becuase I took half hour to type the solution online. That is the how serious I am about this question.

- Chem -
**ur class fella**, Tuesday, February 17, 2009 at 12:51amand forgot to mention, I need a volume of 625mL of pH of 11.65....differnt from the original question posted....