Easy percent composition question

In summary, to find the atomic density of uranium atoms per cm^3 in a solution of water, you can use the mass percentage method. This involves finding the mass percentages of U-235, U-238, and water in the solution and then multiplying them by the solution density. You can then use Avogadro's number to convert to atoms/cm^3. Alternatively, you can use the atomic or mole percentages given in the problem and multiply by the solution density to find the density with respect to each substance. Finally, use Avogadro's number to convert to atomic density.
  • #1
thenewbosco
187
0
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
 
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  • #2
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.
 
  • #3
how would i calculate the mass percentage from the atomic percentage?
 
  • #4
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.
 

1. What is percent composition?

Percent composition is the percentage of each element present in a compound. It is calculated by dividing the mass of each element by the total mass of the compound and multiplying by 100.

2. How do I calculate percent composition?

To calculate percent composition, you need to determine the molar mass of each element in the compound and the total molar mass of the compound. Then, divide the molar mass of each element by the total molar mass and multiply by 100.

3. Why is percent composition important?

Percent composition is important because it helps us understand the chemical makeup of a compound. It allows us to determine the amount of each element present and can also help with identifying unknown substances.

4. Can percent composition be used in everyday life?

Yes, percent composition can be used in everyday life. For example, it can be used to determine the nutritional content of food or the composition of household cleaning products.

5. How is percent composition related to empirical and molecular formulas?

Percent composition is used to calculate the empirical formula of a compound, which is the simplest whole number ratio of the elements present. The molecular formula, which represents the actual number of atoms of each element in a compound, can also be determined using percent composition and the molar mass of the compound.

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