A medical technologist needs to prepare a 5% (m/v) 1L NaCl solution. How many grams of NaCl be weighed out?
Answers
Answer:
This is the exact same approach you use when calculating percentage, the only difference being the fact that you need to multiply the ratio by 1 million, instead of by 100.
So, you can safely assume the density of water to be equal to 1 g/mL. This would make the mass of water equal to
1L⋅1000mL1L⋅1 g1mL=1000 g
Sodium chloride will dissociate completely in aqueous solution to form sodium cations, Na+, and chloride anions, Cl−.
NaCl(aq)→Na+(aq)+Cl−(aq)
The important thing to notice here is that you have a 1:1 mole ratio between sodium chloride and sodium cations. This mole ratio will help you determine the mass of sodium chloride you need to dissolve in order to get this particular solution.
The concentration of the target solution (in ppm) will be
ppm=msolutemwater⋅106
Plug in your values and solve for msolute.
msolute=ppm⋅mwater106
msolute=20⋅1000 g106=0.02 g
This means that your solution must contain 0.02 g of sodium cations, Na+. Use sodium's molar mass to determine how many moles of sodium cations would be present
0.02g⋅1 mole23.0g=0.000870 moles Na+
The aforementioned mole ratio tells you that you must add the same number of moles of sodium chloride to the solution.
0.000870