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There are not any high school level gas law problems that the ChemTeam is aware of that use the Celsius temperature directly in the calculation.
If you have a Celsius temperature in the problem, you MUST change it to Kelvin, in order to use it in your problem.
Sometimes your teacher might put a temperature in the problem, but you really don't need to use it. Your teacher is doing what he or she is driven to do: confuse poor teenage kids. The ChemTeam understands this fully for, you see, this is what happened in his class. (The ChemTeam is retired now.) All teachers know this is really fun! OK, back to work.
You can convert between Celsius and Kelvin like this: Kelvin = Celsius + 273.15. Often, the value of 273 is used instead of 273.15. Check with your teacher on this point. All examples to follow will use 273.
A point before going on to some problems: very often in gas law problems it looks like significant figure rules are being violated. The reality is that they are not. In reality, the significant figure concept is more complex than the simple rules taught at this level.
However, having said that, be careful to watch your teacher's actions in class and ask what the rules are in your class. If I am saying something different than what your teacher wants you to do, please don't use this phrase: "Well, there's some guy on the Internet who says you're wrong."
Last point: never, ever, use a degree sign on the symbol for Kelvin. Do not do this: °K. This is what is known as an 'idiot indicator.' You do that and your study partner will say "Didn't you learn anything in high school? You're an idiot." The ChemTeam will remain silent as to the reasons.
Also, never say 'degrees Kelvin.' This value: 225 K is said "two hundred twenty five Kelvins." Please trust the ChemTeam on these points.
Example #1: convert 25.0 °C to Kelvin.
Answer: 25.0 + 273 = 298.0
Essentially the 273 is being treated as 273.0
Everybody (except you!) in chemistry knows the true conversion value is 273.15, but the decimal portion is usually ignored. Also, the most common type of thermometer in high school labs is readable to the nearest tenth. So temperatures are usually written just to the tenth place at this introductory level
The ChemTeam occasionally had a student who insisted on using 273.15. You may be the same. However, be careful on this. You may do a calculation on a test (or a web-based assignment) using 273.15 and the answer was done with 273. That difference may generate a value for your answer that causes a deduction of points. Follow the policy your classroom teacher sets down.
Example #2: convert 375 K to degrees Celsius.
Answer: 375 − 273 = 102 °C
Example #3: Convert −50 °C to Kelvin. (That's a minus 50.)
Answer: −50 plus 273 = 223 K
Here are some temperature conversion problems (no answers provided):
1. What temperature scale must always be used when working gas law problems? __________
2. The boiling point of water on the Kelvin scale is __________.
3. 0 K is also known as __________.
4. 40.0 °C is what temperature on the Kelvin scale?
5. -20.0 °C is what temperature on the Kelvin scale?
6. 298 K is what temperature in degrees Celsius?
7. What is the temperature (give in both °C and K) at STP?
8. Convert -145.0 °C to K
9. Convert 20.0 °C to K
10. Convert 252 K to °C
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