# If the concentration of #"SO"_2# in air is #"0.5 ppm"#, what is the percent by mass?

##### 1 Answer

This is basically a unit conversion. One way of defining a **part-per-million**, or

#"ppm" = "mg"/"kg"#

since there are

#("0.5 mg SO"_2)/("kg air")#

The **percent by weight**, or weight percent, or

#"%w/w" = (m_("component"))/(m_"total") xx 100%# ,where

#m# is the mass in a given unit, such as#"g"# or#"mg"# , as long as both masses have thesameunits.

We just need to convert one of the units to the other. As the name suggests,

In other words, a *numerically*

#"0.5 parts per million" xx ("1 part per hundred")/("10000 parts per million")#

#= color(blue)(0.00005% "w/w SO"_2 " in air")#