Question

I need help with both of these questions in the picture .

Answer 1

In order to answer question 4 we need to calculate how much 2.56x1024 molecules of water weight.

We know that 1 mol of water molecules (6.022x1023 molecules) weight 18g. This is because according to the periodic table 1 mol of hydrogen weights 1g and 1 mol of oxigen weights 16g. So knowing the chemical formula H2O we calculate:

$1mol_{H2O}=2x1\frac{g}{mol}+1x16\frac{g}{mol}=18\frac{g}{mol}$

Now we need to calculate how much 2.56x1024 molecules weight (mH2O):

$m_{H2O}=\frac{2.56x10^{24}molecules}{6.022x10^{23}\frac{molecules}{mol}}x18\frac{g}{mol}=76.5g$

So we know that the mass of water is 76.5 g.

Now we calculate the total weight, given that the cup weights 5g:

$Total\text{ weight=}5g+76.5g=81.5g$

So the correct answer is B: 81.6g