# Need Help on a Few Chem Problems

1. Aug 13, 2006

### coffee

Just came back to school.. and I realize I've forgotten so many things! ARGG! >_<
So if anyone can, I would really appreciate it you could help and show me how to do these problems asap!

1) An element consists of 1.40% of an isotope with mass 203.973 amu, 24.10% of an isotope with mass 205.9745 amu, 22.10% of an isotope with mass 206.959 amu, and 52.40% of an isotope with mass 207.9766 amu. Calculate the average atomic mass and identify the element.

2) The element rhenium (Re) has two naturally occuring isotopes, 185Re and 187Re, with an average atomic mass of 186.207 amu. Rhenium is 62.60% 187Re, and the atomic mass of 187Re is 186.956 amu. Calculate the mass of 185Re.

3) A compound contains only carbon, hydrogen, and oxygen. Combustion of 10.68 mg of the compound yields 16.01 mg CO2 and 4.37 mg H2O. The molar mass of the compound is 176.1 g/mol. What are the empirical and molecular formulas of the compound.

THANKS!

Last edited: Aug 13, 2006
2. Aug 13, 2006

### sdekivit

let's see what you've tried and then we can give hints that leads you to the correct answer(s)

3. Aug 13, 2006

### PRDan4th

It has been a while since I did problems like this, but for #3 you should do a mass balance to find out how much O2 was required to burn the chemical. Then do a carbon, hydrogen and oxygen balance to create three equations with three unknowns and solve!

4. Aug 13, 2006

### coffee

Oops, sorry for putting it into the wrong section >o<

Um for the first two I'm not really sure but does it has anything to do with multiplying the percent with the amu then add them together? Lol, I really forgot this ..either that or my last teacher taught something simililar to this but it looks different.

As for the 3rd one I think I found some last year notes.. so I'll try it. brb

5. Aug 13, 2006

### mrjeffy321

For the first two questions....
The molar mass of an element is a wieghted average of the molar masses of the element's isotopes.
To find the element's molar mass, find the sum of each isotopes molar mass mulitplied by the percentage abundance (in decimal form).
If you already know the molas mass and one of the two isotope's molas mass, then you can take the above process and work backward.

The third question is a bit of a pain, but to solve it you need to convert the grams of CO2 and H2O into moles of Co2 and H2O.
Also, write out the general form of the equation,
a CxHyOz + b O2 --> c CO2 + d H2O
where a, b, c, and d are the molar coefficients in the reaction and x, y, and z are the subscripts on the Carbon, Hydrogen, and Oxygen in thesubstance being combusted.
Convert things to whole number ratios.
If you can find the propper ratios and know the actual molar mass of substance you can find what factor you need to multiply everything by to get the actual molecular formula.

6. Aug 13, 2006

### coffee

I still don't understand how to solve it. I think I need a work out form instead of a written explaination. Sorry, I'm slow :'( I don't think I've learned this last year either.. maybe something similar but easier.

7. Aug 14, 2006

### sdekivit

for problem 1 & 2:

asumme we have 100 different isotopes.

Then for problem 2 of the 100 Re-atoms, 62.6 atoms are Re-187 and 37.4 atoms are Re-185

The you only need to use the formula you use for every calculation of the average:

$$\bar{a.m.} = \frac {(62.6\cdot185.956) + (37.4\cdot x)} {100} = 186.207$$

And solve for x, the atomic mass of Re-185.

For problem 3 you need to get to know the amount of mol C,H and O in the compound and you must bear in mind that the mass of C and H in the compuond will form CO2 and H2O after the reaction. All that's left on mass of the compund is oxygen.

8. Aug 14, 2006

### coffee

Thanks for helping everyone. A friend and my teacher showed me how to do it. I get it now. :D