Question

To prepare a solution of NaCl, you weigh out 2.634 ± 0.002 g (molecular weight 58.44 ± 0.01 g/mol) and dissolve it in a volumetric flask whose volume is 100.00 ± 0.08 mL. Express the molarity of the solution, along with its uncertainty, with an appropriate number of digits.

Answer 1

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Mass of NaCl = 2.634 ± 0.002 g

Molar mass of NaCl = 58.4430 g/mol

Volume = 100.00 ± 0.08 mL = 0.10000 ± 0.00008 L

Molarity = moles/volume = mass/(molar mass x volume)

= 2.634/(58.443 x 0.1000) = 0.45069555 M

Assuming no uncertainty/error in the molar mass,

%error in molarity = %error in mass + %error in volume

= 100 x 0.002/2.634 + 100 x 0.08/100.00 = 0.16%

Absolute error in molarity = 0.16/100 x 0.45069555= 0.0007 M

So Molarity = (0.4507 ± 0.0007) M

Answer 2

NaCl has a mass of 2.634 0.002 g.

NaCl has a molar mass of 58.4430 g/mol.

Volume = 100.00 ± 0.08 mL = 0.1 ± 0.00008 L

Molarity = =  mass/(molar mass x volume)

= 2.634/(58.443 x 0.1000) = 0.45069555 M

Assuming no uncertainty/error in the molar mass,

%error in molarity = %error in mass + %error in volume

= 100 x 0.002/2.634 + 100 x 0.08/100.00 = 0.16%

Absolute error in molarity = 0.16/100 x 0.45069555= 0.0007 M

So Molarity = (0.4507 ± 0.0007) M

Molarity (M), or molar concentration, is the concentration of a solution measured as the number of moles of solute per liter of solution.

To calculate the molarity of a solution, you divide the moles of solute by the volume of the solution expressed in liters. Note that the volume is in liters of solution and not liters of solvent.