Answer:
Na2S2O8
Step-by-step explanation:
See the attached worksheet. Make the convenient assumption that we have 238 grams of the compound. Then calculate the actual masses of each element. For example, there is (19.3%)*(238g) = 45.94 grams of Na. Now divide that by the molar mass of sodium (23 g/mole) to obtain the actual moles of Na in the 238 gram sample. Na is 2.0 moles.
Do the same for all the elements. We can see from the attached table that there are whole numbers of moles for each of the elements. That very convenient outcome allows us to saw that there are 2 Na, 2 S, and 4 O for each molecule of the compound. The molecular formula is Na2S2O8.
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In the common event that the moles are not all whole numbers, the smallest one should be multiplied by a factor that makes, or keeps it whole, and the same factor is used on the other elements. If all are whole, then stop. The new values represent the molecular formula. For example, if the outcome of this calculation was 1.5, 2, and 2.5, multiply the smallest by 2 to get 3. The others are also multiplied by the same factor, which will yield a result of Na3S4O5.
Values of 1, 2.33 and 4 would be multiplied by 3 (3*2.33 = 7) to yield Na3S7O12, for example.