A sample of helium (He) effuses 2.0 times faster than another gas. What is the molar mass of the other gas? (the correct answer is 16g/mol. I just need an explanation of this.)

The correct answer is 16 gram per mole.

Explanation:

Let A be the gas helium, and B be the unknown gas. It is clearly mentioned in the question that the effusion rate of helium gas is two times more than that of gas B. The molar mass of helium is 4.0 gram per mole. To solve the problem, Grahm's law is used, that is,  

Rate of effusion A/rate of effusion B = √ (Molar mass B/Molar mass of A

2.0 = √Molar mass of B/4.0 gram per mole.  

Now squaring both the sides we get,  

4.0 = Molar mass of B / 4.0

The molar mass of B = 16 gram per mole.  

i believe it is-

d.) gravity pulled particles of dust and gas together to form planets.

the answer for sodium is 0.186

the answer for rubidium is 0.251