alleryStart learning →A flask holding 8.0 x 10^2 g of water is heated from 21 degrees C to 85 degrees C. How much heat does the water absorb, using 4.184 J/g degree C?
AAbout 2.7 x 10^5 J
BAbout 7.0 x 10^4 J
CAbout 3.3 x 10^3 J
DAbout 2.1 x 10^5 J
Answer & Solution
Correct answer: D. About 2.1 x 10^5 J
1. Use q equals c times m times the temperature change.
2. The temperature change is 85 minus 21, which is 64 degrees Celsius.
3. The mass is 8.0 times ten squared grams, that is 800 g.
4. Multiply 4.184 J/g degree C by 800 g, giving 3347.2 J per degree Celsius.
5. Multiply by 64 degrees Celsius, giving about 214,000 J.
6. Reported to two significant figures that is about 2.1 times ten to the fifth joules.
7. Using the final temperature of 85 instead of the change of 64 would wrongly give about 2.8 times ten to the fifth joules.
8. The value is positive because the water gained heat.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.1 Energy Basics_
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