250.0 mg solid magnesium placed in 9.1 g aqueous zinc nitrate... What is the limiting reagent?
I balanced the equation:
Mg (s) + 2HNO3 (aq) -> Mg(NO3)2 (aq) + H2 (g)
250 mg 9.1 g
You may have balanced the equation but it's the wrong equation.
Mg + Zn(NO3)2 ==> Mg(NO3)2 + Zn
I do these the long way.
mols Mg = grams/molar mass = ?
mols Zn(NO3)2 = grams/molar mass = ?
Using the coefficients in the balanced equation, convert mols Mg to mols of either product.
Do the same and convert mols Zn(NO3)2 to mols of the same product.
It is likely that these two values will not agree; the correct value in limiting reagent problems is ALWAYS the smaller value and the reagent producing that value is the limiting reagent.
To determine the limiting reagent in a chemical reaction, you need to compare the moles of each reactant to the stoichiometric ratio in the balanced equation. Let's start by converting the masses of the reactants into moles.
1. Convert the mass of magnesium (250.0 mg) to moles:
- Grams to moles conversion: moles = mass (g) / molar mass (g/mol)
- Molar mass of magnesium (Mg): 24.31 g/mol
- Moles of magnesium = 250.0 mg / 1000 mg/g / 24.31 g/mol
2. Convert the mass of zinc nitrate (9.1 g) to moles:
- Molar mass of zinc nitrate (Zn(NO3)2): 189.36 g/mol
- Moles of zinc nitrate = 9.1 g / 189.36 g/mol
Now that we have the moles of each reactant, we can compare them to determine the limiting reagent.
In the balanced equation: 1 mole of magnesium reacts with 2 moles of nitric acid (HNO3).
3. Calculate the moles of nitric acid (HNO3) needed to react with the moles of magnesium:
- Moles of nitric acid = (moles of magnesium) x (2 moles of nitric acid / 1 mole of magnesium)
Compare the moles of nitric acid and the actual moles provided by the zinc nitrate. Whichever is smaller will be the limiting reagent.
4. Compare the moles of nitric acid available to the moles calculated:
- If the moles of nitric acid available is less than the calculated moles, it is the limiting reagent. Otherwise, magnesium is the limiting reagent.
By following these steps, you'll determine which of the two reactants, magnesium or zinc nitrate, is the limiting reagent in the reaction.