**Step 1**: Calculate the mass of M and O in the compound

**100% = % mass O + % mass M **

**% mass M = 100% - % mass O**

**% mass M = 100% - 16.99% O**

**% mass M = 83.01% M**

Recall that * mass percent* is given by:

$\overline{){\mathbf{\%}}{\mathbf{}}{\mathbf{mass}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}\frac{\mathbf{mass}\mathbf{}\mathbf{of}\mathbf{}\mathbf{X}}{\mathbf{total}\mathbf{}\mathbf{mass}}{\mathbf{\times}}{\mathbf{100}}}$

.

Assuming we have **100 g** of the compound, this means we have

**mass of M = 83.01 g M**

**mass of O = 16.99 g O**.

**Step 2**: Determine the moles of M and O

We are given the formula of the compound which is M_{2}O. This means the lowest whole number ratio of **2 M: 1 O****. **

Now, we need to get the moles of each element in the compound.

$\mathbf{16}\mathbf{.}\mathbf{99}\mathbf{}\overline{)\mathbf{g}\mathbf{}\mathbf{O}}\mathbf{}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{O}}{\mathbf{16}\mathbf{}\overline{)\mathbf{g}\mathbf{}\mathbf{O}}}$** = 1.06 mol O**

Since the lowest whole number ratio of M and O is 2: 1, the number of moles of M is twice the number of moles of O. So

**number of moles of M = 2×number of moles of O**

**number of moles of M = 2.12 mol**

**Step 3**: Determine the molar mass of M

A metal (M) forms an oxide with the formula M_{2} O.

If the oxide contains 16.99 % O by mass, what is the identity of the metal?

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