Answer:
21
Explanation:
Formula;
Mass number = number of protons + number of neutrons
= 9 + 12
= 21
The given question is incomplete. the complete question is:
Calculate the volume in liters of a 0.91 M magnesium fluoride solution that contains 249.2 g of magnesium fluoride . Round your answer to 2 significant digits.
Answer: 4.4 Liters
Explanation:
Molarity of a solution is defined as the number of moles of solute dissolved per liter of the solution.

where,
n = moles of solute
To calculate the moles, we use the equation:

=volume of solution in L

Thus the volume in liters is 4.4 L
The amount of grams of phosphorus that are recommended is 0.700 g
Why?
This is an unit conversion problem. To convert between units we use conversion factors. In this case the conversion factor we want is from mg to g, and this is relatively simple: 1000 mg = 1 g. Now, we arrange the quantities in the following way:

Conversion factors are really useful for converting units! Just remember to put the unit you don't want at the bottom, and the one you want at the top.
Have a nice day!
#LearnwithBrainly