You find a compound composed only of element X and hydrogen, and you know that it is 91.33% element X by mass. Each molecule has 2.67 times as many H atoms as X atoms. What is element X?

Question

Quizplus · Accepted Answer

Since the compound is 91.33% element X by mass, we know that the mass ratio of X to H in the compound is 91.33:8.67. This means that for every 91.33 grams of the compound, 83.99 grams is element X and 7.34 grams is hydrogen (based on the assumption that we have a 100 gram sample).

We also know that each molecule in the compound has 2.67 times as many H atoms as X atoms. This means that we can set up a ratio of H atoms to X atoms in the compound:

H atoms : X atoms = 2.67 : 1

To simplify this ratio, we can divide both sides by the smaller number (which is 1 in this case), giving us:

H atoms : X atoms = 2.67

So for every 2.67 H atoms, there is 1 X atom in the compound.

We can use these ratios to find the atomic mass of element X. Let's say that the atomic mass of X is m. Then the atomic mass of H is 1.008. Using the ratios, we can set up the following equation:

83.99g X / m = 7.34g H / 1.008

Solving for m, we get:

m = 28.06

The atomic mass of element X is 28.06. Looking at the periodic table, the only element with an atomic mass close to 28.06 is silicon (Si), which has an atomic mass of 28.09. Therefore, element X is most likely silicon (choice B).

You Find a Compound Composed Only of Element X and Hydrogen