# Moles, mass and gas calculations

### Moles, mass and gas calculations

#### Lessons

In this lesson, we will learn:
• To recognize the format of stoichiometry test questions and calculations.
• To recall the molar volume of gas at standard temperature and pressure and its meaning.
Methods to calculate number of moles of chemicals in reactions using mass, moles and volume of gas.

Notes:

• The units of mass is g, the units of amount of atoms or molecules is mol, and the units of atomic or molecular mass ($M_R$) is g/mol (pronounced “grams per mole” sometimes written gmol$^{-1}$).

• The formula: $n(mol) = \frac{mass(g)}{M_R (\frac{g}{mol})}$ can be used to calculate the number of moles of a substance when given mass, and using the periodic table to find atomic or molecular mass $M_R$ of that substance.

• With the moles formula above, you can treat the unit terms like you would in general algebra: $\frac{g}{\frac{g}{mol}}=mol$, where g cancels out.

• The molar volume of gas at standard temperature (0°C, 273K) and pressure is 22.4 litres per mole (22.4 L/mol).
The molar volume of gas at room temperature (25°C, 298K) and pressure is 24 litres per mole (24 L/mol).
$\circ$ Whenever answering test questions involving gas volume, check the conditions the reaction is occurring under. DO NOT CONFUSE THE TWO TERMS.

• Stoichiometry calculations involve unit conversions from one quantity given in the question to an unknown quantity:
$\circ$ To get to moles, use the equation and the molar ratios shown.
$\circ$ To get to volume, use the molar volume of gas constants.
$\circ$ To get to mass, use the atomic/molecular masses shown in the periodic table.
• Introduction
Recap of stoichiometry: Introduction
a)
Recap stoichiometry basics; what a chemical equation tells us.

b)
Using number of moles in stoichiometry calculations.

c)
Molar volume: its meaning and use.

d)
A molar volume tip to remember.

• 1.
Calculate the masses and volumes of reactants and products used in chemical reactions.
2C$_6$H$_{14\;(l)} +$19O$_{2\;(g)}$→12CO$_{2\;(g)} +$14H$_2$O$_{\;(g)}$
a)
If 75g of C$_6$H$_{14}$ is burned, what mass of CO$_2$ would get produced from this reaction?

b)
If 240g of H$_2$O is produced from this reaction, how many moles of C$_6$H$_{14}$ would be required?

c)
At STP, what volume of O$_2$ would be required to produce 85 L of CO$_2$ in this process?

d)
What mass of C$_6$H$_{14}$ would be required to produce 15 moles of CO$_2$?

• 2.
Calculate the masses and volumes of reactants and products used in chemical reactions.
P$_{4\;(s)} +$5O$_{2\;(g)}$→P$_4$O$_{10\;(s)}$
a)
At STP, what volume of O$_2$ gas is needed to completely combust 2kg of P$_4$?

b)
If 25 L of O$_2$ were available, how much mass of P$_4$ could be reacted with?

c)
What mass of P$_4$O$_{10}$ would be produced by this?

• 3.
Calculate the volume and number of moles of reactants involved in chemical reactions.
2NH$_{3\;(aq)} +$NaOCl$_{\;(aq)}$→N$_2$H$_{4\;(aq)} +$NaCl$_{\;(aq)} +$H$_2$O$_{\;(l)}$
a)
If 5000 kg of hydrazine (N$_2$H$_{4}$) is required from this industrial process, how much ammonia gas (in L completely dissolved in solution at STP) is required as starting material?

b)
How many moles of hydrazine could be produced if only 5 kg of NaOCl was available?

• 4.
Calculate the volume of reactants required in a chemical process.
SiCl$_{4\;(g)} +$2H$_{2\;(g)}$→Si$_{\;(s)} +$4HCl$_{\;(g)}$
500 mg of Si is required from this process. What is the total volume, in L, of H$_2$ and SiCl$_4$ required to produce this?