# What is molarity of a solution containing 7 g of NaOH in 500 mL of NaOH solution?

##### 1 Answer

#### Explanation:

Molarity is defined as *moles of solute*, which in your case is sodium hydroxide, *liters of solution*.

#color(blue)("molarity" = "moles of solute"/"liters of solution")#

Notice that the problem provides you with the volume of the solution, but that the volume is expressed in *milliliters*,

Moreover, you don't have the number of moles of sodium hydroxide, you just have the mass in grams. So, your strategy here will be to

*determine how many moles of sodium hydroxide you have in that many grams**convert the volume of the solution from milliliters to liters*

So, to get the number of moles of solute, use sodium hydroxide's **molar mass**, which tells you what the mass of **one mole** of sodium hydroxide is.

#7color(red)(cancel(color(black)("g"))) * "1 mole NaOH"/(40.0color(red)(cancel(color(black)("g")))) = "0.175 moles NaOH"#

The volume of the solution in liters will be

#500color(red)(cancel(color(black)("mL"))) * "1 L"/(1000color(red)(cancel(color(black)("mL")))) = "0.5 L"#

Therefore, the molarity of the solution will be

#color(blue)(c = n/V)#

#c = "0.175 moles"/"0.5 L" = "0.35 M"#

Rounded to one sig fig, the answer will be

#c = color(green)("0.4 M")#