Molecular Formula from Mass and Volume Calculations

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Discussion Overview

The discussion revolves around determining the molecular formula of a compound based on its mass composition and volume of gas under specific conditions. Participants explore the relationship between empirical and molecular formulas, particularly in the context of gas laws and stoichiometry.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks assistance in deriving the molecular formula from given mass percentages and gas volume, indicating uncertainty about the process.
  • Another participant explains that equal volumes of gases at identical conditions imply equal mole amounts, suggesting a method to find the moles of hydrogen gas to relate it to the unknown gas.
  • A different participant expresses confusion regarding the concept of equal volume leading to equal mole amounts, indicating a lack of familiarity with this principle.
  • One participant mentions the ideal gas law, stating that one mole of gas occupies a specific volume under standard conditions, which can be used to calculate moles from volume.
  • There is a recognition that the method for obtaining the empirical formula is distinct from that for the molecular formula, with a suggestion that the molecular formula can be derived from the empirical formula using a constant multiplier.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the concepts involved. While some agree on the methods to approach the problem, others question the explanations provided, indicating that the discussion remains unresolved with multiple viewpoints on the process.

Contextual Notes

Some assumptions about gas behavior and the application of the ideal gas law may not be fully articulated, leading to potential gaps in understanding for participants. The discussion does not resolve the steps necessary to transition from empirical to molecular formulas.

Who May Find This Useful

This discussion may be useful for students learning about molecular formulas, gas laws, and stoichiometry, particularly those seeking clarification on the relationship between empirical and molecular formulas in the context of gas calculations.

preet
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What is the molecular formula of a compound which contains 92.3% carbon and 7.7% hydrogen by mass, if 0.39g of its vapour occupies an equal volume of .01g of hydrogen gas under identical conditions?

I don't understand how to do this question. I can figure out the empirical formula... but I don't know how to get the molecular formula. Any help would be appreciated.
 
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This is not very hard, I think. Let's take a look.

I'll start from the second part. An equal volume means equal mole amount at identical circumstances, as you know. You'll just find how many moles are there in 0.01 g of H2 gas, and make it equal to the gas you're asking about.

If 0.39 grams of this gas is equal to the mole amount you'll find, then one mole of this gas is the molecular weight of the gas.

About molecular formulae, try dividing percentage results into their corresponding atomic weights, for example, if a substance gives 20% of carbon, then you'll divide 0.20 to 12. What you'll do next is dividing the numbers into the smallest one to obtain a 1 in the series, and expressing them in terms of relative magnitudes of the smallest one.
 
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I don't really understand what you mean by an equal volume means equal mole amount at identical circumstances...I don't think we've learned that. And in the last part, isn't that to get the empirical formula?
 
[ ignore this, didn't read the entire post]
 
Considering your question involves ideal gases, one mole of gas is known to occupy 22.4 liters in 1 atm ordinary pressure (in 0 °C=273,15 K). So you can directly calculate how many moles are there in a given volume of the gas, by using the known formula, [tex]P \times V=n \times R \times T[/tex]

The last part is used to obtain empirical formula, you are right, and molecular formula can be derived from empirical one, because you will know the molecular formula from the volume calculations, and from there you can multiply the empirical formula with a constant number to obtain molecular formula, I think.
 
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