Homework Help: AP chemistry

Posted by Chem is try on Sunday, October 3, 2010 at 10:37am.

8.3 grams of NaCl and 3 grams of KBr were dissolved in 47 mL of water. What is the mole fraction of KBr in the solution?

I know that the mole fraction of the KBr and Nacl (without using the mL H2O is .2, but, i am confused on where i use that mL in the problem.) Please help!

AP chemistry - bobpursley, Sunday, October 3, 2010 at 10:54am
The wording is a bit ambiguous, to me.

Normally, one would want the mole fraction of the solutes, which would be

fraction=molesKBr/(molesKBr+molesNaCl)

However, the wording is not as specific as this. As it is worded, one has to inclue the moles of water, so
fraction=MolesKBr/(moleswater+molesNaCl+molesKBr)

How do you get moles of water?
moleswater=densitywater * volumewater/molmass water
AP chemistry - Chem is try, Sunday, October 3, 2010 at 11:12am
i got 1.2 but i don't think i did it right. below is my work:
3g(1mole/199g)=.03moles KBr
.03/(1moleH2O+.03molesKBr+.14moles NaCl)= 1.2

moles H2O = (1(47))/47=1
AP chemistry - Chem is try, Sunday, October 3, 2010 at 11:13am
nevermind,i got .02...did i do this correctly??
AP chemistry - bobpursley, Sunday, October 3, 2010 at 11:28am
Your math is right, however, in the 3 grams (one significant figure), I suspect the writer was sloppy, and meant at least 3.00 grams
3.00/199=.00151moles. I dont know how you got .03 out of that.

How did you get moles water as 1?
AP chemistry - Chem is try, Sunday, October 3, 2010 at 11:32am
d=m/v=47g/47mL
because 1g water = 1mL water
moles= dv/m=1(47)/47=1
AP chemistry - bobpursley, Sunday, October 3, 2010 at 11:34am
Nuts to that.

Moles water= masswater/molmasswater=47g/18g/mol= not 1
AP chemistry - Chem is try, Sunday, October 3, 2010 at 11:38am
so, i got 6.82 moles of water and plug that in and get .00361, right?

Answer this Question

Related Questions

Chemistry - Given that the solubility of sodium chloride is 36 grams per 100 ...
CHEM - A student used 10 mL water instead of 30 mL for extraction of salt from ...
chemistry - How many grams of KBr dissolved in 100 g of water is needed to raise...
Chemistry - 80.0 grams of KBr is mixed with sufficient water to prepare 1865 mL ...
science - When substances such as sugar and table salt are dissolved in water, ...
chemistry - If exactly 100.0 grams of NaCl (molecular mass = 58.44 g/mol) are ...
chemistry - What is the percent by mass of 3.55 g NaCl dissolved in 88 g H2O? 2...
science - When substances such as sugar and table salt are dissolved in water, ...
CHEMISTRY - Assume an organic compound has a partition coefficient between water...
CHEMISTRY - Assume an organic compound has a partition coefficient between water...

For Further Reading

First Name:
School Subject:
Answer: