Question

While you were "sweating" your chemistry test, water vapor evaporates from your body, absorbing 50,000 J of energy. (assume no
temperature change). What mass of water evaporates?
A:O 150 g
B:O 22 g
C:11,962 g
D:O 105,000 g

Answer 1

Answer:

$\boxed {\boxed {\sf B. \ 22 \ grams}}$

Explanation:

We need to use the formula for heat of vaporization.

$Q=H_{vap}*m$

Identify the variables.

• The heat absorbed by the evaporating water is the latent heat of vaporization. For water, that is 2260 Joules per gram.

• Q is the energy, in this problem, 50,000 Joules.

• m is the mass, which is unknown.

$H_{vap}=2260 \ J/g\\Q=50,000 \ J$

Substitute the values into the formula.

$50,000 \ J=2260 \ J/g*m$

We want to find the mass. We must isolate the variable, m.

m is being multiplied by 2260 J/g. The inverse operation of multiplication is division. Divide both sides by 2260 J/g.

$\frac{50,000 \ J}{2260 \ J/g} =\frac{2260 \ J/g*m}{2260 \ J/g}$

$\frac{50,000 \ J}{2260 \ J/g} =m$

Divide. Note that the Joules (J) will cancel each other out.

$\frac{50,000 \ }{2260 \ g} =m$

$22.1238938 \ g =m$

Round to the nearest whole number. The 1 in the tenth place tells us to leave the number as is.

$22 \ g \approx m$

The mass is about 22 grams, so choice B is correct.