# A sample of #H_2O# with a mass of 46.0 grams has a temperature of 100 C. How many joules of energy are necessary to boil the water? (Use 2.0934 J/g for the heat of vaporization of water)

##### 1 Answer

Here's what I got.

#### Explanation:

The idea here is that you're dealing with *needed* to convert this sample to vapor at

The problem provides you with the **heat of vaporization** of water

#DeltaH_"vap" = "2.0934 J g"^(-1)#

The heat of vaporization, also called the *enthalpy of vaporization*, tells you the energy needed in order to convert

In this case, you know that **phase change**.

Use this value as a *conversion factor* to calculate the energy needed for your sample

#46.0color(red)(cancel(color(black)("g H"_ 2"O"))) * overbrace("2.0934 J"/(1color(red)(cancel(color(black)("g H"_ 2"O")))))^(color(blue)(=DeltaH_ "vap")) = color(darkgreen)(ul(color(black)("96.3 J")))#

The answer is rounded to three **sig figs**.

**SIDE NOTE** *It's interesting that the problem wants you to use that specific value for the enthalpy of vaporization of water because the actual value is*

#DeltaH_"vap" = "2257 J g"^(-1)#

https://en.wikipedia.org/wiki/Enthalpy_of_vaporization

*This means that the amount of heat that you would actually need is equal to*

#46.0color(red)(cancel(color(black)("g H"_ 2"O"))) * "2257 J"/(1color(red)(cancel(color(black)("g H"_ 2"O")))) = "104,000 J" -> # to three sig figs