How are you finding the moles of the unknown substance when you are only given the grams of the unknown molecular compound?
The Correct Answer and Explanation is:
To find the moles of an unknown substance when you’re given its mass in grams, you need to use the molar mass (molecular weight) of the compound. The molar mass is the mass of one mole of a substance, usually expressed in grams per mole (g/mol). The formula to calculate the number of moles from the mass of the substance is:
[
\text{moles} = \frac{\text{mass in grams}}{\text{molar mass in g/mol}}
]
Step-by-Step Explanation:
- Identify the mass of the substance: You are given the mass of the unknown substance, typically in grams (g).
- Find the molar mass of the substance: This can be calculated by adding the atomic masses of each element in the molecular formula of the compound. For example, for water (H₂O), the atomic mass of hydrogen is approximately 1 g/mol, and the atomic mass of oxygen is approximately 16 g/mol. The molar mass of water is calculated as:
[
\text{Molar mass of H}_2\text{O} = (2 \times 1 \, \text{g/mol}) + (1 \times 16 \, \text{g/mol}) = 18 \, \text{g/mol}
] - Apply the formula: Once you have the mass and the molar mass, plug the values into the formula to calculate moles. For instance, if you have 36 grams of water, you would calculate the moles as follows:
[
\text{moles of H}_2\text{O} = \frac{36 \, \text{g}}{18 \, \text{g/mol}} = 2 \, \text{moles}
]
Why This Works:
The concept behind this calculation is that one mole of any substance contains exactly 6.022 × 10²³ molecules (Avogadro’s number). This number is the same for all substances, but the mass of one mole varies depending on the substance. Using the molar mass allows you to convert between the macroscopic mass of the substance (in grams) and the microscopic number of particles (moles).
In summary, to convert grams of an unknown compound to moles, you divide the given mass by the compound’s molar mass, which gives you the amount of substance in terms of moles.