Can someone help me find the identity of X

Element X, a member of group 5A, forms two chlorides, XCl3 and XCl5. Reaction of an excess of Cl2 with 8.729 of XCl3 yields 13.233 of XCl5. What is the atomic mass of element X?

Balanced equation for reaction:

XCl3 + Cl2 ---> XCl5

Determine mass and moles of Cl2 that reacted:

13.233 - 8.729 = 4.504 g

4.504 g / 70.906 g/mol = 0.063521 mol

From 1:1 molar ratio between XCl3 and Cl2, we know that 0.063521 mol of XCl3 must have reacted.

molar mass of XCl3:

8.729 g / 0.063521 mol = 137.42 g/mol

Subtract out the weight of the three Cl:

137.42 - 106.359 = 31.0

Phosphorus.

Here:

http://www.chemteam.info/Stoichiometry/W…

Problem #18 is another one like your problem.

Cl2 + XCl3 ---> XCl5
Find mass of Cl2 by subtraction:
13.233 g XCl5 - 8.729 g XCl3 = 4.504 g Cl2
convert to moles:
4.504 g / (70.91 g/mol Cl2) = .06352 mol Cl2
since mol Cl2 = mol XCl3 = mol XCl5, choose either XCl3 or XCl5 to calculate molar mass of X.
I'll pick XCl3:
.06352 mol XCl3 * (Z g/mol) = 8.729 g XCl3
Z = 137.43 g/mol XCl3
subtract mass of chlorine to get mass of X:
137.43 g/mol XCl3 - (35.453 g/mol Cl * 3) = 31.06 g/mol X
X is most likely Phosphorus.

its either V which has an atomic mass of 50.9415g or it would be Nb which has an atomic mass of 92.90638g but to be honest im not to sure which one it is =S

I hope that narrowing it down helped :L