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2.40g of the element Z combines exactly with 1.6g of oxygen to form a compound with the formula ZO2. What is the relative atomic mass of Z?
(A) 24.0
(B) 32.0
(C) 48.0
(D) 64.0

Answer
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Hint: We know that 1 mole contains a fixed number of particles and it is the definite amount of a substance which can be expressed in terms of weight, volume and number of particles and we also know that relative atomic mass is the average mass of atoms compared to 112th the mass of carbon-12atom.

Complete Step by step answer:We are aware with the fact that 1 mole contains a fixed number of particles and it is the definite amount of a substance which can be expressed in terms of weight, volume and number of particles and we also know that relative atomic mass is the average mass of atoms compared to 112th the mass of carbon-12 atom.
As given in the question, mass of oxygen is 1.6g and we know the molecular mass of oxygen as well which is 16g, so we can now calculate the moles of oxygen using this:
moles=massmolecularmass
moles=1.616molesofoxygen=0.1mol
From the given formula, ZO2, we know that oxygen combines with element Z in a ratio 1:2, so we can say that number of moles of Z=0.5mol.
Now, using these moles and the given mass of the element we can easily calculate the atomic mass:
moles=massmolecularoratomicmass
0.05=2.4at.mass
atomicmass=2.40.5atomicmass=48g
Or relative atomic mass =48as it is a dimensionless quantity.

Therefore the correct answer is (C).

Note: A relative atomic mass is basically the ratio of the mean of average atomic mass of all atoms of an element in a given sample and it is considered relative to that of carbon-12. That is why relative atomic mass is a dimensionless quantity.
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