How many atoms or ions of each element are in 177.0 g 023 Ba2+ ions

Question

How many atoms or ions of each element are in 177.0 g of the following substances?

 

(a) BaSO₄

 

 

×

 10²³ Ba²⁺ ions

 

 

×

 10²³ SO₄²⁻ ions

 

(b) Mg₃(PO₄)₂

 

 

×

 10²⁴ Mg²⁺ ions

 

 

×

 10²³ PO₄³⁻ ions

 

(c) O₂

 

 

×

 10²⁴ O atoms

 

(d) KBr

 

 

×

 10²³ K⁺ ions

 

 

×

 10²³ Br⁻ ions

Answer 1

This type of problem may seem tough at first, but you'll have it down in just a few steps! I can help guide you through the BaSO₄ portion of the question.

1) Find the molecular mass of the compound.

To do this, for each element in the compound, count the atoms (or ions) of that element in the compound and multiply by that element's average atomic mass (found on the bottom of its periodic table square). Repeat for each element. When you're done with all the elements, sum up the values to get the molecular mass of the compound. For example, using BaSO₄, the molecular mass would be: (137.34 g Ba/mol x 1 Ba) + (23.06 g S/mol x 1 S) + (16 g O/mol x 4 O) = ?

2) How many mols of that compound do you have?

To find this, take your given mass, in this case 177.0 g, and multiply by the reciprocal of the compound's molar mass. (Remember, the molar mass is a set ratio, like 4 quarters in a dollar, so we can change it around for our convenience in problems.) 177.0 g of BaSO₄ x (1 mol/233.36 g BaSO₄) = __?__ mol BaSO₄. This gives us mols because the grams in the numerator (177.0 g) cancel out the grams in the denominator of the molar mass.

3) How many mols of the target atom or ion do you have? This step is important!

We're close! Now, it's time to find the mols of the target atom or ion. To do that, we can use the molecular formula! in BaSO₄, there is only 1 Ba. So, if we had one mol of BaSO₄, we'd have one mol of Ba²⁺ atoms. If we were looking to find the number of O atoms present, we'd notice that there are 4 Oxygens in one BaSO₄. So, if we had one mol of BaSO₄, we'd have FOUR mols of oxygen atoms!

____(answer from 2)___ mol BaSO₄ x (1 mol Ba²⁺ / 1 mol BaSO₄) = _____ mol Ba²⁺

4) Multiply by Avogadro's number

Last step, and it's an easy one!

Finally, we have the mols of the atom or ion we're looking for! Now, all we have to do is multiply that by Avogadro's number which tells us there's 6.02x10²³ atoms or ions in one mol.

It should look something like this: ___ mol Ba²⁺ x (6.02x10²³ ions Ba²⁺ / 1 mol Ba²⁺) = ____ ions of Ba²⁺

Nice work! You can apply these steps to each part of your question! Hope this helps :)

Answer 2

Hello, Alyssa,

I'm not quite sure what the questions are for a few of these, but I'll help with the first and hopefully you'll see how the process works and how it might be applied to the other questions.

For the Barium Sulfate, we need to molar mass. I get 233.4 g/mole. Since 1 mole = 6.02E23, We need 233.4 grams of BaSO4 to have that many molecules. We have less than that, so we have fewer than 6.02E23 molecules. To get the exact number, just divide the actual amount by the molar amount to give numbers of molecules:

(177.0 g/(233.4 g/mole) to give us moles BaSO4. Multiply that figure by 6.02E23 to give molecules of BaSO4. That will be the number of Ba and S atoms, but multiply it times 4 to get the number of O atoms.

Bob