A substance having a molecular (formula) weight of 58 is:?

Question

1) CaSO4
2) HCl
3) KNO3
or
4) Mg(OH)2

Answer 1

Magnesium hydroxide

Answer 2

Look at a periodic table. There are two numbers next to each element: one is an integer and one is a decimal that is about twice the size of the element. That second number is the atomic mass of that element.

For example, the atomic mass of magnesium (Mg) is 24.3050, hydrogen (H) is 1.000794 and oxygen is 15.9994.

In Mg(OH)2, there is one Mg, 2 O and 2 H so we do

24.3050+(2x15.99954)+(2x1.000794)

The answer is 58.305388, which is close enough to 58 for government work. You can look up the atomic masses of all the other elements involved and check the others (which would be good practice), but none of them work out to anything close to 58 (HCl is 36.453494 etc).

Atomic mass is how heavy the average atom of a given type is. For example, all carbon atoms have six protons. Some carbon atoms have six neutrons (carbon 12) while others might have seven or eitht (carbon 13, carbon 14). Scientists will take the average of all the weights of the atoms in a random chunk of carbon using a device called a mass spectrometer, and call that the atomic mass. In the case of carbon most carbon atoms have six protons and six neutrons in the nucleus so the average is close to 12 (12.011).

There will never be a molecule that weighs exactly 58.305388 atomic mass units, but your average Mg(OH)2 will weigh that amount assuming you started with magnesium and oxygen and hydrogen with the same percent of different isotopes (different numbers of neutrons).

Did that help?

Answer 3

If you have any idea of what the molecular weight is, you should be able to calculate the molecular weight of eachof these compounds and answer it for your self.

If you don't know how to do that, you need to learn. Just look up the atomic mass of each element in the compound, multiply them by their subscripts, and add them all together...