The products will be magnesium phosphate and potassium chloride. You then have to watch a solubility chart to see which one of these is not soluable. In this case it is magnesium phosphate.
According to the task, you are proveded with patial pressure of CO2 and graphite, and here is complete solution for the task :
At first you have to find n1 =moles of CO2 and n2 which are moles of C
<span>The you go :
</span>

n1 n2 0
-x -x +2x

After that you have to use the formula

Then you have to solve x, and for that you have to use <span>RT/V
And to find total values:</span>

I am absolutely sure that this would be helpful for you.
The metalloid that has three valence electrons is Boron~
Answer:
Molecular Evidence.
It's C, the amount of protons in the nucleus of each atom :)