Unit 1 Chemistry 122

Question

When a sample of food is burned, it caused the temperature of the water in a calorimeter containing 780 g of water to increase by 8.9•C. Calculate the amount of heat gained by the water. What is the mass of the sample of it was 100% protein? (Protein = 4.0 kcal/g?

Answer 1

Hello, Chloe, (again)

Unlike the other problem, this wants us to work with calories, instead of Joules. Both calories and Joules are a measure of energy, or heat. Here, we want to find the mass of a protein sample based on the amount of heat released from combustion of the sample. We can calculate the amount released based on how much the temperature rises of water in a calorimeter (a vessel designed to minimize heat lossto the surroundings).

The specific heat of water expressed in calories is 1.00 cal/g*C. The temperature of 780 grams of water rose by 8.9^oC in this analysis. The total heat produced is calculated with the expression:

q = C*M*(DT), whre C is the specific heat, M the mass, and DT the temperature change.

Here, we have:

q = (1.0 cal/gC)*(780 grams)*(8.9C)

q = (1.0 cal/gC)*(780 grams)*(8.9C)

q = 6942 calories, or 6.942 kcal

The size of the protein sample that produced 6.942 kcal can be calculated from a knowledge of its calorie content per gram. In this case we have 4.0 kcal/gram. To find grams protein that will contain 6.942 kcal, do the follwing division:

(6.942 kcal)/(4.0 kcal/gram) = 1.74 grams protein

1.74 grams of protein has enough energy to raise 780 grams of water by 8.9^oC.

Bob