Chemistry Convert the following quantities:

Question

8.87 x 10^27 P4O10 molecules ---->  _____ atoms

 

1.129 x 10^20 formula units PbCl4 ----> _______ mol ions

Answer 1

8.87x10²⁷ molecules P₄O₁₀ x ( 14 atoms / 1 molecule P₄O₁₀ ) = 1.24x10²⁹ atoms

 

For every molecule of P₄O₁₀, there are 14 atoms (four phosphorus, ten oxygen). By taking your number of molecules and multiplying by the conversion factor in parentheses of 14 atoms for every molecule, you get the answer above. I used strike through to show how the units cancel. 

 

1.129x10²⁰ FU PbCl₄ x ( 5 ions / 1 FU PbCl₄ ) x ( 1 mol ions / 6.022x10²³ ions ) = 9.374x10^-4 mol ions

 

Conversions in this online help are a little hard to follow because I can't clearly show the fractions. When you're doing these problems on paper, write out the conversion factors in parentheses as upright fractions, numerator over denominator. That allows you to see much more easily how units on top of the fraction cancel with units on the bottom of the next fraction.

 

You're starting with 1.129x10²⁰ FU PbCl₄ and need to find how many mol of ions are present. Each PbCl₄ is comprised of five ions (1 Pb⁴⁺, 4 Cl^-), which gives the conversion factor in the first set of parentheses. Units cancel, and you have now converted to ions. In order to convert a number of ions, molecules, atoms, etc to mol, you must use Avogadro's number, 6.022x10²³. Every 6.022x10²³ ions gives you 1 mol of ions, yielding the fraction in the second set of parentheses. The unit of ions cancel, leaving you with the answer in mol ions.