Set the formula for copper oxide if the mass ratio of elements is 8: 1.

Let’s count the total number of parts:
8 + 1 = 9 pieces;
The mass fraction of copper is 8/9, and the mass fraction of oxygen is 1/9.
Let us denote: a is the number of copper atoms, b is the number of oxygen atoms, Mr is the molecular weight of the compound. The relative atomic mass of copper is 64, oxygen is 16.
Let’s compose the following expressions:
8/9 = 64 * a / Mr;
1/9 = 16 * b / Mr;
Let us express the molecular weight from the expressions:
Mr = 64a / (8/9) = 72a;
Mr = 16b / (1/9) = 144b;
Let’s equate the results obtained: 72a = 144b. This means that a = 2, b = 1.
Answer: The formula for copper oxide is Cu2O.