Question

How many atoms of phosphorus are in the compound phosphorus trichloride?
A. 1
B. 2
C. 3
D. 4

Answer 1

This is a chemistry question, Please post it in the chemistry group instead of biology.
For the question, the name of phosphorus trichloride already gives the answer.
In chemistry the numerical prefixes before the elements in the name are the amount of atomes in the compound.
Since there's no prefix for phosphorus we can assume it is 1. for chloride it would be 3 since there's tri before chloride.

1= mono
2= di
3= tri
4= tetra
5= penta
6= hexa
7= hepta
8= octa
9= nona
10 = deca

You could write the compounds as mono-phosphorus tri-chloride. \(\sf\large P_1Cl_3\)
So for every molecule there would be 1 phosphorus atom and 3 chlorine atoms.

Cl - P - Cl
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Cl