Determine the volume of oxygen

In summary: But do not use rounded values in subsequent calculations.In summary, the conversation discusses the process of determining the volume of oxygen gas needed to completely combust 3.5 moles of propane in a barbecue at standard pressure and 28.0°C. The conversation includes a discussion of using the standard mole ratio approach and the relevant equation, as well as the use of significant figures in calculations. The final answer of 440L is reached by using full precision in all intermediate results, rather than rounding down.
  • #1
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Homework Statement



Determine the volume of oxygen gas required to completely combust 3.5mole of propane in a barbecue at standard pressure and 28.0°C.

Homework Equations



##PV = nRT##

The Attempt at a Solution



My first question is, is the work I've done here still correct? http://gyazo.com/3a22ee323ec1edff0ea071255cf698ac

I'm a bit confused here. If i take the standard mole ratio route, I get a completely different answer. I would find that 17.5mole of oxygen would be needed for 3.5mole of propane. Then using the relevant equation I get 437.72925L = 400L because 100kPa only has 1 significant digit. This answer is clearly very far off.

This leads me to believe the question is intending me to do the work as I've shown in the image above.

Why does this incredibly different answer occur? Is it merely a fault of significant error? Or am i wrong completely and the mole route is appropriate (which really wouldn't make sense)?
 
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  • #2
If it is a standard pressure, 100 has at least three significant figures, if not infinitely many. Doesn't matter much, as 3.5 has only two and it is what limits number of digits on the final answer.

I don't see any difference between both approaches. I got 438 L in both cases, rounded down to 440 L. You got different final results as you rounded down intermediate results - never do that. Round down only the final result, use full precision during calculations.
 
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  • #3
Borek said:
If it is a standard pressure, 100 has at least three significant figures, if not infinitely many. Doesn't matter much, as 3.5 has only two and it is what limits number of digits on the final answer.

I don't see any difference between both approaches. I got 438 L in both cases, rounded down to 440 L. You got different final results as you rounded down intermediate results - never do that. Round down only the final result, use full precision during calculations.

Ah so I would keep the volume calculation at ##88L##. I thought because 100 had only 1 sig fig it would change the answer dramatically.

Doing that I get 440L directly by using the ratio 1/88 = 5/z.
 
  • #4
No idea where you got 88 L from - unless it is already rounded 87.637. And I just told you to not round down intermediate results, haven't I?
 
  • #5
Borek said:
No idea where you got 88 L from - unless it is already rounded 87.637. And I just told you to not round down intermediate results, haven't I?

Yes you're correct. I did round it.

I have a question now though just for clarity. Is it technically wrong to use significant figures in every step of a problem however many calculations there may be?
 
  • #6
Yes, it is technically wrong. Round down only the final result, use full precision (or at least several so called guard digits) on all intermediate results. Otherwise you are introducing hard to detect errors (you have just witnessed such a situation).

Note - if you are REPORTING intermediate results, report them rounded.
 
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Related to Determine the volume of oxygen

1. How is the volume of oxygen determined?

The volume of oxygen can be determined by measuring the amount of oxygen present in a given space or container. This can be done using various methods such as gas chromatography or titration.

2. What is the SI unit for measuring oxygen volume?

The SI unit for measuring oxygen volume is cubic meters (m3). This is the standard unit for measuring volume in the International System of Units.

3. Can the volume of oxygen change?

Yes, the volume of oxygen can change depending on the temperature, pressure, and presence of other gases in the given space. This is known as the ideal gas law, which states that the volume of a gas is directly proportional to its temperature and inversely proportional to its pressure.

4. How does oxygen volume affect living organisms?

Oxygen volume plays a crucial role in the survival of living organisms as it is essential for cellular respiration. This process allows organisms to convert oxygen and nutrients into energy, which is needed for various life processes.

5. What are the potential hazards of working with oxygen volume?

Oxygen is a highly reactive gas and can pose a fire hazard in high concentrations. Additionally, oxygen-depleted environments can also be dangerous for humans as it can cause asphyxiation. It is important to handle oxygen volume with caution and follow proper safety protocols.

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