Question

14 g of an element X combines with 16 g of oxygen. On the basis of this, which of the following statements is correct?
[At. Wt of oxygen = 16]
A.The element X could have an atomic weight 7 and its oxide formula is X O.
B.The element X could have an atomic weight 14 and its oxide formula is ${{\text{X}}_{2}}{{\text{O}}_{3}}$.
C.The element X could have an atomic weight 7 and its oxide formula is${{\text{X}}_{2}}\text{O}$.
D.The element X could have an atomic weight 14 and its oxide formula is X${{\text{O}}_{2}}$.

Answer 1

Hint:Atomic weight of oxygen is 16g. The element X combines with oxygen has a weight of 14g so either its atomic weight is 7 or 14. So on forming the oxide, it will have two possibilities- one will be formation of oxide having a single mole of each i.e. when atomic weight of X is 14 because one mole of an element has weight equal to its atomic weight. The other possibility will be when the atomic weight of X is 7 then two moles of X will be used in order to form the oxide.

Complete answer:
If atomic weight of X is 14 then the formula for oxide will be XO, in this oxide one mole of X and one mole of oxygen is present having 14g and 16g weight respectively.
If the atomic weight of X is 7 then two moles of X will be used in order to form the oxide because the amount given for X is 14g. So, the formula for oxide will be${{\text{X}}_{2}}\text{O}$. In both the formulas only one mole of oxygen is used because the element X combines with 16g of oxygen and 16 being its atomic weight it has only a single mole. So, the formula for oxide will be either XO or ${{\text{X}}_{2}}\text{O}$ depending on the atomic weight of X being 14g or 7g respectively.
So, out of the given four options option (B) and option (D) can’t be considered at all because these have three and two moles of oxygen which are not possible. In option (A) a single mole of X is present but the atomic weight of X is 7 which is also not true according to the above explanation.

So, the correct choice is option (C) where oxide formed will be ${{\text{X}}_{2}}\text{O}$and the atomic weight of X will be 7.
Note:
Study the periodic table in order to know about the atomic numbers and atomic weights of the elements. There is always one mole present for the element if its weight is given equal to its atomic weight. The subscripts used in the formula for oxide for different elements represents their no. of moles used in order to form that oxide. One can easily obtain the mole ratio for elements combining to form the given oxide, like in ${{\text{X}}_{2}}\text{O}$ the mole ratio will be $2:1$.i.e. 2 moles of X and 1 mole of O combines to form this oxide.