Calculate the molarity of each solution 47.2mg NaCl in 145.3mL of solution
Express your answer using three significant figures.
I like to use the step-wise approach over the conversion pathway approach to answer this question.
Molarity is calculated by dividing the number of moles of the solution by the volume of the solution (M=n/v; where M= molarity, n= # of moles, and v=volume [in L, liters]).
First, the mass NaCl is in mg, which must be converted to grams and the volume is in milliliters, which must be converted to Liters.
To convert to grams, 47.2mg NaClx1g NaCl/10^3g NaCl= 0.0472g NaCl
To convert to liters, 145.3mL NaClx1L NaCl/10^3mL NaCl.
Now, find the molar mass and/or molecular weight (MM and/or MW) of NaCl, by adding their MM(/MW) together. These can be found on the periodic table. The molar mass of Na is 22.990 g/mol; Cl is 35.453 g/mol. When added together, you get 58.443 g/mol NaCl.
Now, the number of mols NaCl must be calculated, like so:
0.0472g NaClx1mol Nacl/58.443g NaCl=0.000807625 mol NaCl
Now, using the molarity formula, M=n/v we get
[NaCl]=0.000807625 mol NaCl/0.1453L
[NaCl]=0.00555324 M NaCl
Now we look at sig figs, and the final answer becomes: 5.56x10^-3 M NaCl, or 0.00556 M NaCl, both answers are correct.