What is the molarity of a salt solution made by dissolving 240mg of NaCl in 4ml of water? Assume final volume is the same as the volume of water.

2 Answers
Jun 17, 2017

Answer:

#"1 mol L"^(-1)#

Explanation:

In order to find the molarity of the solution, you must determine the number of moles of solute present in exactly

#"1 L" = 10^3# #"mL"#

of solution. Notice that you already know the number of milligrams of sodium chloride, the solute, present in #"4 mL"# of solution, so start by calculating the mass of solute present in #10^3# #"mL"# of solution.

#10^3 color(red)(cancel(color(black)("mL solution"))) * "240 mg NaCl"/(4color(red)(cancel(color(black)("mL solution")))) = 60 * 10^3# #"mg"#

Since

#color(blue)("1 g" = 10^3color(white)(.)"mg"#

you can say that #10^3# #"mL"# of solution contain

#60 * color(blue)(10^3color(white)(.)"mg") = 60color(white)(.)color(blue)("g")#

of sodium chloride. Now, to convert this to moles, use the molar mass of the compound

#60 color(red)(cancel(color(black)("g"))) * "1 mole NaCl"/(58.44color(red)(cancel(color(black)("g")))) = "1.027 moles NaCl"#

Since this represents the number of moles of sodium chloride present in #10^3# #"mL"# of solution, you can say that the molarity of the solution is equal to

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

The answer must be rounded to one significant figure, the number of sig figs you have for the volume of the solution.

Jun 17, 2017

Answer:

M=#"1 mol/L"#

Explanation:

The molarity of a solution is the concentration of moles of solute per liter of solution, #"mol/L"#. The mass of #"NaCl"# must be converted to grams and then moles. The volume of the solution must be converted to liters.

Convert #"mg NaCl"# to #"g NaCl"#.

#"1 g=1000 mg"#

#240color(red)cancel(color(black)("mg NaCl"))xx(1"g")/(1000color(red)cancel(color(black)("mg")))="0.24 g NaCl"#

Determine moles of #"NaCl"#.

Divide the mass of #"NaCl"# by its molar mass by multiplying by its inverse.

Molar mass #"NaCl=58.44 g/mol"#

#0.24color(red)cancel(color(black)("g NaCl"))xx(1"mol NaCl")/(58.44color(red)cancel(color(black)("g NaCl")))="0.0041 mol NaCl"#

Molarity of #"NaCl"#.

Convert #"4 mL"# to liters.

#"1 L=1000 mL"#

#4color(red)cancel(color(black)("mL"))xx(1"L")/(1000color(red)cancel(color(black)("mL")))="0.004 L"#

Divide mol NaCl by volume of solution.

#"M"=(0.0041"mol NaCl")/(0.004"L solution")="1 mol/L"# rounded to one sig fig due to #4# mL