# 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).
• 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:
• To get to moles, use the equation and the molar ratios shown.
• To get to volume, use the molar volume of gas constants.
• 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?