Question Video: Determining the Identity of Two Elements Given the Chemical Formula and Relative Formula Mass of a Compound | Nagwa Question Video: Determining the Identity of Two Elements Given the Chemical Formula and Relative Formula Mass of a Compound | Nagwa

# Question Video: Determining the Identity of Two Elements Given the Chemical Formula and Relative Formula Mass of a Compound Chemistry • First Year of Secondary School

A scientist is given the chemical formula and relative formula mass of two unknown compounds, as shown below: Chemical formula XY₂ M_(r) = 64, Chemical formula XY₃ M_(r) = 80. If the atomic mass of element X is twice that of element Y, which of the following are elements X and Y? [S = 32, O = 16, Ni = 59, Si = 28, C = 12, Mg = 24, Ne = 20, Ar = 40, Li = 7, N = 14]. [A] X = Ni, Y = Si [B] X = S, Y = O [C] X = Li, Y = N [D] X = Ne, Y = Ar [E] X = C, Y = Mg

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### Video Transcript

A scientist is given the chemical formula and relative formula mass of two unknown compounds, as shown below: chemical formula XY2, M r equals 64; chemical formula XY3, M r equals 80. If the atomic mass of element X is twice that of element Y, which of the following are elements X and Y? The relative atomic mass of sulfur is 32, oxygen is 16, nickel is 59, silicon is 28, carbon is 12, magnesium is 24, neon is 20, argon is 40, lithium is seven, and nitrogen is 14. (A) X equals Ni; Y equals Si. (B) X equals S; Y equals O. (C) X equals Li; Y equals N. (D) X equals Ne; Y equals Ar. Or (E) X equals C; Y equals Mg.

To answer this question, we need to determine the identity of elements X and Y by comparing their relative atomic mass to the relative atomic masses given. But in the question, we aren’t provided with the relative atomic mass of X and Y. But we are provided the relative formula mass, also called the relative molecular mass, of two compounds that contain only elements X and Y. The relative formula or molecular mass is a ratio of the average mass of one molecule or formula unit to one twelfth of the mass of a carbon-12 atom. The relative formula mass of a molecule or formula unit can be calculated by summing the relative atomic masses of the constituent atoms.

For example, a molecule of carbon dioxide contains one carbon atom and two oxygen atoms. The relative atomic mass of carbon is 12, and the relative atomic mass of oxygen is 16. So the relative molecular mass of carbon dioxide will be equal to 12 plus two times 16. We multiply 16 by two because there are two atoms of oxygen in the chemical formula. So the relative molecular mass of carbon dioxide is 44. Let’s apply this understanding to the first unknown compound. As we don’t know the relative atomic mass of X and Y, let’s define these as the variables X and Y for simplicity.

We know that the relative formula mass of XY2 will be equal to the relative atomic mass of X plus two times the relative atomic mass of Y. We also know that the relative formula mass of this compound is 64. This gives us the equation 64 equals X plus two Y. We are also told that the atomic mass of element X is twice that of element Y. So we can set up a second equation. The relative atomic mass of X equals two times the relative atomic mass of Y. Since we now have a relationship for the value of X, we can substitute this into the first equation. This gives us 64 equals two Y plus two Y. We can combine like terms then divide both sides of the equation by four. This gives us a value of 16 for Y. We can then substitute this value into the second equation, giving us X equals two times 16, or X equals 32.

Now that we know that the relative atomic mass of X is 32 and the relative atomic mass of Y is 16, we can compare these values to the values given in the question. We can see that a relative atomic mass of 32 corresponds to the element sulfur and a relative atomic mass of 16 corresponds to the element oxygen. Therefore, element X must be sulfur and element Y must be oxygen. So from the relative formula mass and chemical formulas of two unknown compounds, we have determined that the elements X and Y are X equals S and Y equals O, answer choice (B).

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