Hello, Kaylee,
Let's establish the molar ratio of the two elements in the salt. Yes, we all know it is NaCl, or one chlorine for every one sodium. But just in case a teacher might give you a trick question, or if something new happens, use the data to insure the molar ratio is correct. The molar masses for Na and Cl are 23 and 35.5, respectively. We are told that 1 gram of chlorine combines with 1.54 g of sodium. What we want to know is how many atoms that is of both elements. We'll use moles to find that. Take each mass and divide by it's molar mass:
Na: 1g/(23g/mole) = 0.0435 moles Na
Cl: 1.54g/(35.5g/mole) = 0.0434 moles Cl
The numbers have to be in a whole unit ratio (no partial atoms allowed), so the data supports a 1:1 ratio of Na to Cl. Aha. NaCl.
To determine the mass of sodium required to react with 45.0 grams of chlorine, we'll use moles again.
Moles Cl = 45.0 grams Cl/(35.5g/mole) = 1.27 moles Cl (3 sig figs).
BUT, chlorine is a diatomic gas. So if we assume the chlorine is provided as the gas, Cl2, we need to use a molar mass of 71.0 for Cl2, not the 35.5 g/mole as shown above.
That also means we need to use this balanced equation:
Cl2 + 2Na = 2NaCl
It tells us we need 2 moles of Na for every 1 mole of Cl2.
Using this assumption, the moles of chlorine is calculated as:
45 g Cl2 / (71 g/mole) = 0.634 moles Cl2. We need twice that number of moles of Na, or 1.28 moles Na.
To get that in grams, use the molar mass:
1.28 moles Na * (23 g/mole) = 29.2 grams Na
I hope this helps. Practice using mole calculations, and keep the units in your work. It is an important, and often used, conversion factor in the sciences.
