Question

Depending on the reaction conditions, elements X and Y react by a weight ratio of either 1 part of X and 1.982 parts of Y, or 1 part of X and 5.946 parts of Y, forming two different compounds. Which of the following pairs of substances is inconsistent with the above data?
(a) X3 Y and X3Y3
(b) X3Y2 and X3Y6
(c) XY and X3Y
(d) XY and X3Y3
(e) X3Y2 and XY2

Answer 1

Answer:

(c) and (d)

Explanation:

The law of definite proportions states that elements in compounds always have a fixed atomic ratio independent of any conditions.

In the context of this problem, we only know the mass ratio of X to Y, but we don't know the molar ratio, so we cannot derive the empirical formula.

However, we notice that in the second compound, mass of Y increased by a fraction of:

$\frac{5.946}{1.982} = 3$

Since mass is directly proportional to moles, in the second compound we should have three times larger number of atoms of Y for the same number of atoms of X (1 part in each case lets us assume this).

a. Notice that X is kept constant and Y increases by a factor of 3, so it's consistent.

b. Similarly to (a), X is kept constant and Y increases by a factor of 3, so it's consistent.

c. Here, Y is kept constant. X increases by a factor of 3. This is inconsistent with the data, since if we start with $XY$, then we'd have $XY_3$ as our second pair, since we should have three times greater number of Y atoms in the second structure.

d. The molar ratio here in each pair is the same, XY is the empirical formula for $X_3Y_3 = (XY)_3$. This means it's inconsistent, as here both formulas have the same ratio of X to Y.

e. This is consistent, since for a constant Y, in the second structure we should have one third of X atoms to obtain the 3 : 1 ratio.