# Question bdd38

Jun 25, 2015

Here's how you'd go about solving these questions.

#### Explanation:

Part A

To get from grams of a substance to moles you need to use molar mass. Molar mass tells what the exact mass of 1 mole of a substance is.

In the case of potassium chloride, its molar mass is listed as 74.55 g/mol. This means that 1 mole of potassium chloride has a mass of 74.55 g.

Since you're dealing with miligrams, mg, you need to convert the mass to grams first, then use the compound's molar mass.

6.34cancel("mg") * (1cancel("g"))/(1000cancel("mg")) * "1 mole KCl"/(74.55cancel("g")) = "0.000085 moles KCl"

If you want, you can express this value in milimoles

0.000085cancel("moles") * "1000 mmoles"/(1cancel("moles")) = "0.085 mmoles KCl"

Part B

To go from mass to number of molecules, you need to go to moles first, then use Avogadro's number.

According to Avogadro's number, 1 mole of any substance contains exactly $6.022 \cdot {10}^{23}$ atoms or molecules of that substance.

So, to go from molecules to grams, go to moles first, then use ammonia's molar mass.

20cancel("molecules") * "1 mole"/(6.022 * 10^(23)cancel("molecules")) = 3.32 * 10^(-23)"moles"

then

3.32 * 10^(-23)cancel("moles") * "17.03 g"/(1cancel("mole")) = 5.7 * 10^(-22)"g" $N {H}_{3}$

Part C

Once again, use Avogadro's number and the molar mass of gold.

5cancel("pg") * (1 cancel("g"))/(10^(12)cancel("pg")) * (1cancel("mole"))/(196.97cancel("g")) * (6.022 * 10^(23)"atoms")/(1cancel("mole")) = 1.5 * 10^(8)"atoms"#

I'll leave parts D and E to you as practice. Don't forget that you get from

• moles to grams and vice versa by using molar mass
• mass/moles to atoms/molecules and vice versa by using Avogadro's number