Relating Mass of a Compound to Mass of a Constituent Element: Sodium in Sodium Chloride

Quantities in Chemical Reactions

Relating Mass of a Compound to Mass of a Constituent Element

In chemical calculations, it is often necessary to determine the mass of a specific element present in a given mass of a compound. This process involves using molar masses and mole ratios derived from the chemical formula.

• Key Concept: The mass of an element in a compound can be found by converting the mass of the compound to moles, using the compound's molar mass, then using the mole ratio from the chemical formula, and finally converting moles of the element to grams using its molar mass.

Example Problem: Find the mass of sodium in 15 g of NaCl

• Step 1: Write the chemical formula and identify the constituent elements. NaCl contains sodium (Na) and chlorine (Cl).

• Step 2: Calculate the molar mass of NaCl. Sodium (Na): 22.99 g/mol Chlorine (Cl): 35.45 g/mol Molar mass of NaCl = 22.99 g/mol + 35.45 g/mol = 58.44 g/mol

• Step 3: Set up the conversion factors to relate grams of NaCl to grams of Na.

  • Convert grams of NaCl to moles of NaCl: $\frac{15\,\text{g NaCl}}{58.44\,\text{g NaCl}}$

  • Use the mole ratio from the formula: $1\,\text{mol NaCl} : 1\,\text{mol Na}$

  • Convert moles of Na to grams of Na: $1\,\text{mol Na} = 22.99\,\text{g Na}$

• Step 4: Combine the conversion factors into a single calculation: $15\,\text{g NaCl} \times \frac{1\,\text{mol NaCl}}{58.44\,\text{g NaCl}} \times \frac{1\,\text{mol Na}}{1\,\text{mol NaCl}} \times \frac{22.99\,\text{g Na}}{1\,\text{mol Na}}$ $= 5.9\,\text{g Na}$

• Step 5: State the answer. There are 5.9 g of sodium in 15 g of NaCl.

General Formula

To find the mass of an element in a given mass of a compound:

• $\text{Mass of element} = \text{Mass of compound} \times \frac{1\,\text{mol compound}}{\text{Molar mass of compound}} \times \frac{n\,\text{mol element}}{1\,\text{mol compound}} \times \frac{\text{Molar mass of element}}{1\,\text{mol element}}$

• Where n is the number of moles of the element per mole of compound (from the chemical formula).

Additional info:

• This type of calculation is fundamental in stoichiometry and is widely used in chemical analysis and laboratory work.

• Always check the chemical formula to determine the correct mole ratio between the compound and the element.