Acompound is 42% sodium, 19% phosphorus, and 39% oxygen by mass. what is the empirical formula of this compound? na3po4 napo na9p4o8 na4p3o

Take the one with the smallest percentage, 19% phosphorus and and divide all the other percentages by this number.
**Round to nearest 0.0, 0.25, 0.5, 0.75

P = 19/19 = 1 ---> 1
O = 39/19 = 2.05 --> 2
Na = 42/19 = 2.21 --> 2.25

0.25 = 1/4 So Multiply by 4 to get all ratios to integer values

P = 1×4 = 4
O = 2×4 = 8
Na = 2.25×4 = 9

Answer → Na9P4O8