Need help soving this GChem question

[p0l0bear01]

HOW MANY ml OF WATER MUST BE ADDED TO 65ml OF A 5.5 m SOLUTION OF NaOH IN ORDER TO
PREPARE A 1.2-M NaOH SOLUTION
A. 230ml
B. 235ml
C. 229 ml
D. 1L
E. NONE OF THE ABOVE

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[RozhonDDS]

HOW MANY ml OF WATER MUST BE ADDED TO 65ml OF A 5.5 m SOLUTION OF NaOH IN ORDER TO
PREPARE A 1.2-M NaOH SOLUTION
A. 230ml
B. 235ml
C. 229 ml
D. 1L
E. NONE OF THE ABOVE

E

 

[allstardentist]

i get 206ml using calc. not 100% sure though.

 

[madaboutmed]

using d formula
n1m1v1=n2m2v2
(1)(65)(5.5)=(1)(1.2)(V2)
we get
V2= 65 *5.5/1.2
V2=297 ml( approx)
hence volume of water 2 b added 297-65= 232mlans
(NOT SURE BUT mAYB ITS RIGHT)

 

[allstardentist]

i dont think thats right because it is 5.5 m not 5.5 M.

 

[keibee82]

E.

5.5 moles NaOH / 1 kg H2O * 1 kg/1000g * 1g H2O/1ml H2O * 1000ml/1L
= 5.5 moles NaOH/1L

(5.5M)(0.065L) = (1.2M)V

V = 298ml

298 ml - 65ml = 233ml

correct me if i'm wrong

 

[dontbam]

i got 232ml, i'm guessing he meant 5.5M not molality? that's my guess

 

[dental#1]

I was also thrown by the molality. I tried it M1V1=M2V2 and subtracted but I don't think its correct. Molality is mol/kg (m).

Molality
Molality (m) is defined as the number of moles of solute per kilogram of solvent.

molality = moles of solute / kilograms of solvent

Although their spellings are similar, molarity and molality cannot be interchanged. Compare the molar and molal volumes of 1 mol of solute dissolved in CCl4 (d = 1.59 g / mL)

By definition, a 1 M solution would contain 1 mol of solute in exactly 1.00 L of CCl4 , and a 1 m solution would contain 1 mol of solute in 629 mL of CCl4 .

1 kg of CCl4 x (1000 g / 1 kg) x (mL / 1.59 g) = 629 mL CCl4

 

[dental#1]

E.

5.5 moles NaOH / 1 kg H2O * 1 kg/1000g * 1g H2O/1ml H2O * 1000ml/1L
= 5.5 moles NaOH/1L

(5.5M)(0.065L) = (1.2M)V

V = 298ml

298 ml - 65ml = 233ml

correct me if i'm wrong

Thats what I was thinking!!!!!!!!!!!!!!!! I believe you are right in the conversion. Thats what I kept getting...... but I wanted to be sure.

 

[allstardentist]

how does 5.5 moles NaOH/1L solvent all of sudden become 5.5M? 5.5 M means 5.5 moles of solute/L of SOLUTION(solvent + solute) not solvent.

 

[dental#1]

molarity Definition:
A unit of concentration expressing the number of moles of solute dissolved per liter of solvent.


http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-M/molarity.html

 

[dental#1]

Definition of molarity :

A unit of concentration expressing the number of moles of solute dissolved per liter of solvent


http://www.everythingbio.com/glos/definition.php?ID=1797

 

[allstardentist]

uhhh.. noo thats not correct. Look at the kaplan book. Molarity is moles of solute/vol of SOLUTION. this is how i learned it since i was in high school. whatever source u got there is wrong or is not the standard definition.

 

[dying2badentist]

Definition of molarity :

A unit of concentration expressing the number of moles of solute dissolved per liter of solvent


http://www.everythingbio.com/glos/definition.php?ID=1797

yeh he is right, it's not moles of solute/liters of sovent. it's moles of solute/liters of solution. molality is kg of solvent

 

[Dentalist]

5.5 molarity = 5.5 mol / kg = 5.5 mol / L

To find out how many moles of NAOH there are,

5.5 mol / L * (0.065)L = 0.3575 mol

We want the solution to be 1.2 M = 1.2 mol / L

Hence, let X = the volume of water we need to add

1.2 mol / L = 0.3575 mol / (0.065 L + X)

0.065 + X = 0.3575 / 1.2

X = 0.3575 / 1.2 - 0.065 = 233 ml

 

[allstardentist]

65ml of 5.5 m NaOH solution does not contain 5.5 mol of NaOH. It is much lesser.

 

[dental#1]

I see what you mean..... I thought maybe the Kaplan was incorrect (it has been before) I went digging around a little more and found exactly what you guys are saying. I looked it up also in my oooooooooold chem book I had to dig for.

The molality of a solution is calculated by taking the moles of solute and dividing by the kilograms of solvent.
The molarity of a solution is calculated by taking the moles of solute and dividing by the liters of solution.


http://education.yahoo.com/reference/dictionary/entry/molarity

 

[dental#1]

So whats your final answer allstar?

 

[p0l0bear01]

It is 5.5m not Molarity, I haven't been able to solve the answer. By looking at some of the answer response it is close to the actual answer of 235mL. Choice B. So please explain this problem as clearly as possible. Thanks for the responeses.

 

[allstardentist]

This is what i did, not 100% sure if its right,

5.5 m of NaOH solution = 5.5 moles of NaOH/1000g H20= 5.5 moles of NaOH/55.6mol of H2O. From this, we can say that the mole fraction of H2O in the solution is 55.6/(55.6+5.5)= .91.

So I multiplied 65ml by .91 to get the vol. of water in the 65ml solution.(this is the part im not sure about because the density of NaOH is not given. so this is the only way i could think of). 65ml x.91 gives us 59ml of solvent(H2O).

So now we can use the molality. 59ml of solvent x 5.5 mol NaOH/1000ml solvent (b/c density of water is 1g/1ml) to get .325 mol of NaOH in the 65 ml solution.

And finally, .325mol/(x+.065)L = 1.2 M, x = .206L or 206 mL. Like I said before. This is my best guess.