How Do I Solve These Chemistry Isotope and Compound Problems?

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Discussion Overview

The discussion revolves around solving chemistry problems related to isotopes and compounds, specifically focusing on calculating average atomic masses and determining empirical and molecular formulas. The problems presented involve both theoretical calculations and practical applications in chemistry.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant requests help with three specific chemistry problems involving isotopes and combustion analysis.
  • Another participant suggests reviewing the user's attempts before providing hints to guide them toward the answers.
  • A different participant proposes using mass balance and creating equations based on the combustion reaction to solve the third problem.
  • One participant expresses uncertainty about the method for calculating average atomic mass, recalling a possible approach involving multiplying percentages by atomic mass units (amu).
  • Another participant explains that the molar mass of an element is a weighted average of its isotopes' molar masses and outlines a method for solving the problems.
  • A participant mentions needing a more structured workout form rather than written explanations to understand the problems better.
  • One participant provides a formula for calculating the atomic mass of an isotope based on the average atomic mass and the abundance of the isotopes.
  • Another participant emphasizes the importance of determining the amounts of carbon, hydrogen, and oxygen in the compound for the combustion problem.
  • A later reply indicates that the original poster has received help and now understands how to approach the problems.

Areas of Agreement / Disagreement

The discussion contains multiple competing views on how to approach the problems, with no consensus reached on a single method. Participants express varying levels of understanding and different strategies for solving the problems.

Contextual Notes

Some participants express uncertainty about their prior knowledge and the methods required to solve the problems, indicating potential gaps in understanding or missing assumptions about the chemistry concepts involved.

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Just came back to school.. and I realize I've forgotten so many things! :cry: 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 occurring 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! :redface:
 
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let's see what you've tried and then we can give hints that leads you to the correct answer(s)
 
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!
 
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
 
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.
 
I still don't understand how to solve it. I think I need a work out form instead of a written explanation. Sorry, I'm slow :'( I don't think I've learned this last year either.. maybe something similar but easier.
 
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.
 
Thanks for helping everyone. A friend and my teacher showed me how to do it. I get it now. :D
 

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