Hey guys, I have some chemistry homework I need help with!

Question

A sample of Carbon Tetrachloride has a mass of 13.0g. Calculate the following quantities.

 

A). Moles of carbon tetrachloride

 

B.) Number of carbon tetrachloride molecules

 

C.) Number of C atoms

 

D.) Moles of Cl

Answer 1

Hello, Tucker!

 

To start the problem, we must first identify the molecular formula for carbon tetrachloride.

 

Since carbon has no prefix, we will assume one atom.  The prefix for chloride, however, is tetra-, which if we think back is 4.  Does CCl₄ make sense based on their valence electrons?  Carbon has 4 valence electrons, and chlorine has 7.  That means, in order to have a full shell, carbon can either give away or receive 4 electrons.  Chlorine is far more likely to receive 1 electron versus give away 7.  If we have 4 valence electrons from carbon, then we would need 4 chlorine atoms to receive 1.  So, this molecular formula, CCl_4, makes sense.

 

A.)  To find how many moles are in 13.0g of carbon tetrachloride, we will need to convert using one of the conversion factors we know has units with g/mol.  We will therefore need the molar mass of our molecule.  For this, you will need to look at a copy of the periodic table.

 

C has an atomic mass of 12.011 g/mol.

Cl has an atomic mass of 35.45 g/mol.

 

To find the molar mass of the whole molecule, we will need to calculate the following, since we have more than one atom of Cl per molecule.

 

12.011 g/mol + 4(35.45 g/mol) = 153.81 g/mol

 

Now that we have the molar mass for carbon tetrachloride, we can begin  to calculate the moles in 13.0g using a basic proportion.

 

13.0g CCl₄/ X mol = 153.81 g / 1 mol CCl₄

 

X = 0.0845 mol of CCl₄

 

B.) Next, it asks us to find how many molecules of carbon tetrachloride we have in 13.0g.  Thankfully, we already did a lot of the work in step A.

 

The other constant we need is Avogadro's number: 6.022 x 10²³ atoms/mol.

 

We take our answer from the last part....

 

(0.0845 mol of CCl₄) * (6.022 x 10²³ molecules/mol) = 5.089 x 10²² molecules of CCl₄

 

C.)  This may seem like a headache to calculate, but it's actually very simple.  There is one carbon atom in every carbon tetrachloride molecule, which means that there is an equal number of carbon atoms to carbon tetrachloride molecules.

 

5.089 x 10²² atoms of C.

 

D.) You can apply the same principle from C to figure out the answer for D.  For every mol of carbon tetrachloride, there are 4x as many chlorine atoms.  Therefore, the amount of moles you can find by multiplying the moles of CCl₄ by 4 and solving.

 

4 * 0.0845 mol of CCl₄ = 0.338 mol of Cl