Question

You have 1.20 x 10 24 molecules of H2O. What is the mass of this sample?

Answer 1

Answer:

$35.86~g~H_2O$

Explanation:

In this case, we have to remember that relationship between the avogadro's number and the "mol" concept:

$1~mol=6.023x10^2^3~molcules$

With this in mind, we can do the first conversion:

$1.20x10^2^4~molecules~of~H_2O\frac{1~mol_H_2O}{6.023x10^2^3~molcules~of~H_2O}=1.99~mol_H_2O$

Now, if we calculate the molar mass of $H_2O$. The atomic mass of O is 16 g/mol and the atomic mass of H is 1 g/mol, so:

(16*1)+(1*2)= 18 g/mol

In other words, $1~mol~H_2O=~18~g~H_2O$, so:

$1.99~mol_H_2O\frac{18~g~H_2O}{1~mol~H_2O}=35.86~g~H_2O$

I hope it helps!