What is the percent by mass of sodium in na2so4?

32.37%; that is done with sig figs as well. What you want to do is find the molar mass of the entire substance (142.05g/mol) and the mass of the sodium (45.98g). Divide 45.98/142.05. You get .3236888... Which when rounded for sig figs is .3237, which is of course 32.37%. 

m=9

step-by-step explanation:

7m+8=71 subtract   8 from 71 which is 63 than divide it by 7m   equal 9.

                             

High-energy and unstable.