An element X forms an iodide (XI3) and a chloride (XCl3) The Iodide is quantitatively converted to the chloride when it is heated in a steam of chlorine.

2XI3+3Cl2-------2XCl3+3I2 I is the iodide X is the unknown substance Cl is chlorine

If 0.5000g of XI3 is treated, 0.2360g of XCl3 is obtained.

A. Calculate the atomic weight of the element X

B. Identify element X

PLEASE HELP!!!!

## Answers (1)

Let the relative atomic mass of X (Ar) = m

The relative formula mass (Mr) of XI3 = m + (3*127)

The relative formula mass (Mr) of XCl3 = m + (3*35.5)

For every two moles of XI3 you get two moles of XCl3

So, mole ratio = 1:1

As moles = mass/Mr, and moles of XI3 = moles of XCl3

0.5000/(m+381) = 0.2360/(m+106.5)

Solving this equation for m gives

m = 138.8

So, looking up on a periodic table, element X is lanthanum, La, (which is intuitively possible as La has 3 outer electrons and is therefore likely to form a trihalide)