Question

In a laboratory experiment, pure lead metal reacted with excess sulfur to produce a lead sulfide compound. The following data was collected:

Mass of empty evaporating dish: 25.000 g
Mass of evaporating dish and lead metal: 26.927 g
Mass of evaporating dish and lead sulfide: 27.485 g

Solve for the empirical formula of the lead sulfide compound. Be sure to show, or explain, all of your calculations

Answer 1

@ganeshie8

Answer 2

@Chem&Stuff ?

Answer 3

To find the empirical formula of any compound, you need to find the ratio of moles of each element in the compound. In order to find moles, you need to know the mass of the substances you're dealing with.

Given this problem you have all you need.

First step: figure out the masses.
it says the evaporating dish is 25.000g (keep significant figures in mind for every calculation you do.)
the evaporating dish + the lead metal ONLY is 26.927g. meaning there is 1.927g of the lead metal alone. (26.927g-25.000g)
Now, the evaporating dish plus the metal SULFIDE is 27.485. Meaning that if you subtract the mass of the dish and the lead, you're left with the mass of the sulfur. so...
27.485g-26.927g = 0.558g Sulfer..

so now you have : Pb - 1.927g , S - 0.558g

Second step: Find the moles of each substance.

Let's take the lead first...
1.927g Pb
Using the periodic table, find our how many grams per mole lead has... (g/mol).
You should find that Pb has 207.2 g/mol.
Knowing the grams of Pb you have AND the g/mol, you can figure out how many moles of the lead you have.

1.927g x (1mol Pb/207.2gPb) = 0.009300mol Pb or 9.3x10^-3molPb

Do the exact same thing for Sulfer and you should get 1.74molS

Final Step: Fine the ratio of moles
now you have 1.74 x10^-2 molS , and 9.300 x10^-3 molPb

Always keep the lesser of the two (or 3 or 4 or however many elements within the compound) on the bottom of the ratio.

so we have:
1.74 x 10^-2 molS/9.300 x10^-3molPb = 1.9S/1Pb or a 2:1 ratio... of Sulfur to Lead..

Therefore the empirical formula should be PbS2

Answer 4

omg thank you:) soooo much!!!

Answer 5

All good. Use this method when you're not given the full chemical reaction like in this problem. There are sometimes more complicated ways to use ratio's of coefficients within a given reaction, but always ratio of moles to find empirical formula.

Answer 6

ok i will remember that and write it in my notes thank you for the help!