The molecular mass of a compound is 63u. If it contains one hydrogen atom and three oxygen atoms, the atomic mass of the remaining element is:

The atomic mass of the remaining element is 14u, which corresponds to Nitrogen in the compound HNO3. The combined mass of Hydrogen and Oxygen: Mass of H atom = 1x1u = 1u; Mass of 3 O atoms = 3x16u = 48u. Subtotal = 49u. Subtract the known mass from the total molecular mass to find the mass of the remaining element (X): Total mass - known mass; X = 63u - 49u = 14u. An atomic mass of 14 corresponds to Nitrogen (N). Therefore, the compound is Nitric Acid (HNO3).