A sample of seawater contains 34.5 g sodium chloride per liter. To the correct number of significant figures, how many milligrams of sodium chloride are in 75.0 mL of the
sample?

94.2 mg

Explanation:

You can actually use multiple approaches to solve this problem, but they revolve around a couple of unit conversions.

One way to approach this is to use the given concentration to figure out how many milligrams of sodium chloride you get per milliliter of solution.

This will allow you to find the number of milligrams of sodium chloride present in  

15.0 mL

of this solution, provided that you use the conversion factors

1 g

=

10

3

mg

and  

1 L

=

10

3

mL

So, if your solution contains  

6.277 g

of sodium chloride per liter, you can say that it contains

6.277

g

L

⋅

1

L

10

3

mL

=

6.277

⋅

10

_

3

g/mL

This will be equivalent to

6.277

⋅

10

−

3

g

mL

⋅

10

3

mg

1

g

=

6.277 mg/mL

You now know that

6.277 g/L  

=

6.277 mg/mL

So, if  

1 mL

of this solution contains  

6.277 mg

of sodium chloride,  

15.0 mL

will contain

15.0

mL solution

⋅

the given concentration



6.277 mg NaCl

1

mL solution

=

94.155 mg NaCl

Rounded to three sig figs, the number of sig figs you have for the volume of the sample, the answer will be

m

N

a

C

l

=

94.2 mg NaCl

Answer link

Explanation: