1.3 Reacting Masses and Volumes
In the study of chemistry, understanding the relationships between reacting masses and volumes is essential for predicting the outcomes of chemical reactions and optimizing processes. This topic covers the principles and calculations needed to determine the quantities of reactants and products in chemical reactions.
Reacting Masses
Moles and Particles
The concept of the mole is fundamental in chemistry for quantifying substances. One mole is defined as $6.022 \times 10^{23}$ particles (Avogadro's number). It's important to be precise about the type of particles when dealing with moles:
- Molecules: One mole of $H_2O$ contains $6.022 \times 10^{23}$ water molecules.
- Atoms: One mole of $O_2$ contains $2 \times 6.022 \times 10^{23}$ oxygen atoms.
- Ions: One mole of $NaCl$ contains $6.022 \times 10^{23}$ sodium ions and $6.022 \times 10^{23}$ chloride ions.
Calculating Reacting Masses
To calculate the masses of reactants and products in a chemical reaction, the following steps are followed:
- Write the Balanced Chemical Equation: This provides the stoichiometric ratios of the reactants and products.
- Determine the Molar Masses: Calculate the molar mass of each substance involved.
- Calculate Moles of Reactants: Use the mass of the reactants and their molar masses to find the number of moles.
- Use Stoichiometry: Apply the stoichiometric ratios from the balanced equation to determine the moles of products.
- Calculate Masses of Products: Convert the moles of products back to masses using their molar masses.
Example Calculation:
Calculate the mass of magnesium oxide ($MgO$) formed by completely burning 6.0 g of magnesium ($Mg$) in oxygen ($O_2$).
Step 1: Write the balanced equation: $$ 2Mg(s) + O_2(g) \rightarrow 2MgO(s) $$
Step 2: Determine the relative atomic masses:
- Magnesium ($Mg$): 24.31
- Oxygen ($O$): 16.00
Step 3: Calculate the moles of magnesium used: $$ \text{Moles of } Mg = \frac{\text{Mass of } Mg}{\text{Molar mass of } Mg} = \frac{6.0 , \text{g}}{24.31 , \text{g/mol}} = 0.247 , \text{mol} $$
Step 4: Use stoichiometry to find moles of $MgO$: From the balanced equation, 2 moles of $Mg$ produce 2 moles of $MgO$. Therefore, 0.247 moles of $Mg$ will produce 0.247 moles of $MgO$.
Step 5: Calculate the mass of $MgO$: $$ \text{Mass of } MgO = \text{Moles of } MgO \times \text{Molar mass of } MgO = 0.247 , \text{mol} \times 40.31 , \text{g/mol} = 9.96 , \text{g} $$
