easy percent composition question


by thenewbosco
Tags: composition, percent
thenewbosco
thenewbosco is offline
#1
Sep22-08, 09:41 PM
P: 188
1. For a problem I require the atomic density of uranium atoms per cm^3 in a solution of water. The atomic fraction of natural uranium to water is 1 per 100 and the net density of the solution is 1 g/cm^3. (Natural uranium is assumed to be U-235 at 0.72% of uranium atoms and the rest is U-238) (assume the atomic weights of U-235, U-238 and water are 235, 238 and 18 respectively)



What I require is the atomic density of U-235 atoms, the atomic density of U 238, and H2O atoms all in atoms/cm^3.

I have tried to calculate the overall atomic weight of the solution weighted by the atom concentration as in:

0.000072(235) + 0.009928(238) + 0.99(18) = 20.35 g/mol?

then since the solution has net density 1 gm/cm^3:

1 / 20.35 = 0.049 mol/cm^3

then using avogadro's number convert to atoms/cm^3, then use the 1 in 100 again to find how many are U235, U238, water....is this a correct method. something seems wrong in this.

thanks for the help
Phys.Org News Partner Science news on Phys.org
Going nuts? Turkey looks to pistachios to heat new eco-city
Space-tested fluid flow concept advances infectious disease diagnoses
SpaceX launches supplies to space station (Update)
GCT
GCT is offline
#2
Sep22-08, 10:34 PM
Sci Advisor
HW Helper
GCT's Avatar
P: 1,769
A simpler method is to find the mass percentages from the atomic or mole percentages that are given in the problem , then simply multiply this fraction to the solution density to find the density with respect to each substance. Then use Avogadros number to convert to the atomic density.
thenewbosco
thenewbosco is offline
#3
Sep22-08, 10:48 PM
P: 188
how would i calculate the mass percentage from the atomic percentage?

GCT
GCT is offline
#4
Sep23-08, 10:34 AM
Sci Advisor
HW Helper
GCT's Avatar
P: 1,769

easy percent composition question


Assume 1 g of mixture

MW U 235 X moles U 235 + MW U 238 X moles U 238 + MW H2O X moles H2O = 1 g

Assume x is the moles of U 235

(x) X 9928 / 72 = moles of U 238

(x) X 990000 / 72 = moles of H2O

Solve for x

Then simply use Avogadros number to convert to atoms and since you found x you are able to find the moles of U 238 and H2O.


Register to reply

Related Discussions
A question involving self-composition. Calculus 2
percent hydrolysis/percent ionazation Biology, Chemistry & Other Homework 1
Easy gauss' law question that I wanna make sure I didn't make too easy Advanced Physics Homework 0
Need help calculating percent composition Chemistry 4