# Calculate the concentration of an ammonia #(M_M = "17 g/mol")# solution that is #15%# #"NH"_3# and has a density of #"0.90 g/mL"# ?

##### 1 Answer

Here's what I got.

#### Explanation:

Let's start by picking a sample of this ammonia solution that has a volume of

#"1.0 L" = 1.0 * 10^3 quad "mL"#

Use the **density** of the solution to convert its volume to *mass*.

#1.0 * 10^3 color(red)(cancel(color(black)("mL solution"))) * "0.90 g"/(1 color(red)(cancel(color(black)("mL solution")))) = "900. g"#

Now, you know that the solution is **by mass**. This tells you that every

In this case, your sample will contain

#900. color(red)(cancel(color(black)("g solution"))) * "15 g NH"_3/(100color(red)(cancel(color(black)("g solution")))) = "135 g NH"_3#

Next, use the **molar mass** of ammonia to convert the mass to *moles*.

#135 color(red)(cancel(color(black)("g"))) * "1 mole NH"_3/(17color(red)(cancel(color(black)("g")))) = "7.94 moles NH"_3#

As you know, the **molarity** of a solution tells you the number of moles of solute present in exactly **moles** of ammonia, the solute, for every

This means that the molarity of the solution is

#color(darkgreen)(ul(color(black)("molarity = 7.9 mol L"^(-1))))#

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