AP Chemistry - Help with Molecular Weight and Molar Mass

What is the mass of 25 iron atoms?

$2.32*10^{-21}g$

$2.5*10^{-22}g$

$1.23*10^{-23}g$

$2.5*10^{-21}g$

$1.85*10^{-23}g$

Explanation

First, convert the number of atoms to moles using Avogadro's number:

$25\ \text{atoms}*\frac{1\ \text{mole}}{6.022*10^{23}\ \text{atoms}}=4.1514*10^{-23}\ \text{moles Fe}$

Next, convert moles of iron to grams of iron using the molar mass:

$4.1514*10^{-23}mol\ Fe*\frac{55.85g\ Fe}{1mol\ Fe}=2.32*10^{-21}g$

Blood contains various dissolved ions including calcium , which is involved in many biological processes (like muscle contraction). One liter of blood from an unknown animal was shown to contain $6.00*10^{16} atoms$ of . Calculate the mass of calcium atoms in one liter of blood.

None of these

$9.96*10^{-8}$

$3.75*10^{-6}$

$4*10^{-8}$

$3.99*10^{-7}$

Explanation

The answer is not present, but $6.00*10^{16} atoms$ equates to $3.99*10^{-6}g\hspace{1mm}Ca$ .

Use Avogadro's number to convert from atoms to moles:

$6.00*10 ^{16}atoms\hspace{1mm}Ca* \frac{1mol\hspace{1mm}Ca}{6.022* 10^{23}atoms\hspace{1mm}Ca}=9.96*10^{-8}mol\hspace{1mm}Ca$

Convert from moles of to grams of :

$9.96*10 ^{-8}mol\hspace{1mm}Ca\times \frac{40.08g\hspace{1mm}Ca}{1mol\hspace{1mm}Ca}={3.99*10^{-6}g\hspace{1mm}Ca}$

Consider the following reaction:

$N_{2} + 3H_{2} \rightarrow 2NH_{3}$

How much ammonia will be created if $\small 50g$ of nitrogen gas is used in the reaction?

Explanation

Since we are given the mass of nitrogen gas that will be used, we will need to convert the amount into moles. We can then compare the molar ratios in the balanced reaction, and multiply the moles of ammonia by its molar mass. Using dimensional analysis, we can use a calculation that will allow us to end with "grams of ammonia" as the solved unit.

$50 g N_{2} * \frac{1molN_{2}}{28gN_{2}} =molesN_2$

$50 g N_{2} * \frac{1molN_{2}}{28gN_{2}} * \frac{2moles NH_{3}}{1 molN_{2}} =molesNH_3$

$50 g N_{2} * \frac{1molN_{2}}{28gN_{2}} * \frac{2moles NH_{3}}{1 molN_{2}} * \frac{17gNH_{3}}{1molNH_{3}} = 61 gNH_{3}$

What is the atomic number of calcium?

Explanation

The atomic number is equal to the number of protons in an atom and can be determined using the periodic table. The atomic number is located on the top left of the box containing the desired chemical symbol. The chemical symbol for calcium is Ca and the atomic number is 20.

What is the mass in grams of $7.5*10^{15} \textup{ atoms}$ of ?

$7.3*10^{-7} \textup{ g}$

$1.25*10^{-8} \textup{ g}$

$4.40*10^{{-17}} \textup{ g}$

$36\textup{ g}$

Explanation

To determine the mass in grams of $7.5*10^{15} \textup{ atoms}$ of nickel, we first need to convert atoms to moles. To do this we need to know that $6.02*10^{23} atoms = 1 mole$ . Then we will convert moles to grams using the atomic mass of nickel, which can be found on the periodic table. The atomic mass of is $58.7\frac{g}{mol}$ .

Now we have all of the information we need to perform the conversion:

$7.5atoms*\frac{1 mole}{6.02*10^{23}atoms}*\frac{58.7g}{1 mole}=7.3*10^{-7}g$

How many atoms of sulfur are in $4.00\textup{ g}$ of ?

$7.50*10^{22}\textup{ atoms}$

$7.73*10^{25}\textup{ atoms}$

$0.125\textup{ atoms}$

$2.41*10^{24}\textup{ atoms}$

Explanation

The conversion of grams to atoms is a two-step process. First, we need to convert grams to moles using the molecular mass of sulfur. The molecular mass of sulfur can be found on the periodic table and is equal to $32.1\frac{g}{mol}$

Then we will convert moles to atoms using our knowledge that $6.02*10^{23} atoms = 1 mole$ . Using all this information we will perform the following conversion:

$4.00g(\frac{1mol}{32.1g})(\frac{6.02*10^{23} atoms}{1 mol}) = 7.50*10^{22} atoms$

How many moles of potassium are there in $4.3\textup{ g}$ of potassium?

$0.11\textup{ mol}$

$168.13\textup{ mol}$

$39.1\textup{ mol}$

$4.3\textup{ mol}$

Explanation

To determine the number of moles in of potassium, we need to convert grams to moles. To convert grams to moles we need to determine the atomic mass of potassium using the periodic table. The atomic mass of potassium is $39.1\frac{g}{mol}$ .