# A sample of a mineral contains #26.83%# #"KCl"# and #34.27%# #"MgCl"_2#. Find the percentage of chlorine in this sample?

##### 1 Answer

#### Explanation:

Your strategy here will be to use the given percentages to figure out how much chlorine is present in a given sample of this unknown mineral.

To make the calculations easier, let's pick a sample of this mineral that has a mass of

#"26.83% KCl in 100.0 g mineral" \ -> \ "26.83 g KCl"# #"34.27% MgCl"_2 \ "in 100.0 g mineral" \ -> \ "34.27 g MgCl"_2#

Now, use the **molar mass** of potassium chloride and the **molar mass** of *atomic chlorine* to calculate how many grams of chlorine are present in this sample.

#26.83 color(red)(cancel(color(black)("g KCl"))) * (1color(red)(cancel(color(black)("mole KCl"))))/(74.5513color(red)(cancel(color(black)("g KCl")))) * (1 color(red)(cancel(color(black)("mole Cl"))))/(1color(red)(cancel(color(black)("mole KCl")))) * "35.453 g"/(1color(red)(cancel(color(black)("mole Cl")))) = "12.759 g Cl"#

So, you know that this sample contains **molar mass** of magnesium chloride and the **molar mass** of *atomic chlorine*.

#34.27 color(red)(cancel(color(black)("g MgCl"_2))) * (1color(red)(cancel(color(black)("mole MgCl"_2))))/(95.211color(red)(cancel(color(black)("g MgCl"_2)))) * (2color(red)(cancel(color(black)("moles Cl"))))/(1color(red)(cancel(color(black)("mole MgCl"_2)))) * "35.453 g"/(1color(red)(cancel(color(black)("mole Cl")))) = "25.522 g Cl"#

At this point, you know that the sample contains

You can thus say that the **total mass** of chlorine present in the sample is

#"12.759 g" \ + \ "25.522 g" = "38.281 g"#

So, you have a sample of this unknown mineral that has a mass of **percent composition** of chlorine in the sample is

#color(darkgreen)(ul(color(black)("% composition" \ = \ "38.28% Cl")))# This is the case because, by definition, the

mass percentof an element in a substance is given by the mass of said element present in exactly#"100 g"# of the substance.

The answer is rounded to four **sig figs**, the number of sig figs you have for your data.