General Chemistry 1

Question

A chemist carefully measures the amount of heat needed to raise the temperature of a 252.0mg sample of C2H2Cl2 from −3.2°C to 15.9°C. The experiment shows that 5.31J of heat are needed. What can the chemist report for the molar heat capacity of C2H2Cl2? Round your answer to 3 significant digits.

Answer 1

Hello, Madison,

The molar heat capacity, or specific heat expressed in moles, has the units of Energy (Joules, here), mass (mg, here, but grams and moles are more common), and temperature change (generally metric, C or K).

A specific heat of 4.18 J/gK for water, says we need "4.18 Joules to raise 1 gram of water by 1 degree K."

In this case we find that 5.31J is able to raise the temperature of 252.0 mg of C2H2Cl by 19.1 degrees C.

The molar heat capacity has units of J/(mole*K). We have the units of C and J, but not moles. Convert 252mg of C2H2Cl2 into moles by dividing the mass by the molar mass. The relative difference in C and K willl remain the same, so we have 19.1 degrees K.

252mg/(138.9g/mole)

Note we need to convert mg to grams for this conversion.

0.252g/(138.9g/mole)= 0.00037 moles

Then put the data together: Molar heat capacity = 5.31J/(0.00037mole*19.1K)

Molar heat capacity = 717 J/mole*K

This seems too high, so I may have ade a calculation error. I have tio leave, so please check my work.

Bob