SAT Subject Test Chemistry,10 edition

oxygen. To find the ratio between these two elements in the compound, divide the mass of
arsenic by the atomic weight of arsenic, 74.7 grams/mol, and the mass of oxygen by the atomic
weight of oxygen, 16 grams/mol. This will give the mole ratio between arsenic and oxygen in
the compound. To convert to a more easily useful ratio, divide both by the lowest number of
the two, in this case arsenic. This gives us a ratio of 1 mole of arsenic to 2.5 moles oxygen,
which is better stated by doubling both numbers and getting 2 moles of arsenic to 5 moles of
oxygen. This would correspond to the formula in choice D, As 2 O 5.