What is the energy value

Question

A 2.5 g sample of a pretzel is burned completely. The heat it gives off is used to heat 45 g of water from 22 °C to 51 °C. What is the energy value of the pretzel in kcal/g? 

Answer 1

We must first find the number of calories it takes to heat 45 g of water from 22º to 51º

q = mC∆T

q = heat in calories = ?

m = mass = 45 g

C = specific heat of water = 1 cal/gº

∆T = 51º - 22º = 29º

q = (45 g)(1 cal/gº)(29º) = 1305 cal

Now we convert this to kcal:

1305 cal x 1 kcal/1000 cal = 1.305 kcal

Finally we convert this to kcal/g:

1.305 kcal/2.5 g of pretzel = 0.522 kcal/g = 0.52 kcal/g (2 significant figures)