# Observing a Limiting Reactant

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Observing a Limiting Reactant
An experiment was carried out to predict the limiting reactant in a chemical reaction between Magnesium and Hydrochloric acid, using the mole concept.
Limiting Reactant:
It is the reactant that will deplete or will be used up first during a chemical reaction. Limiting reactant also determine how long the reaction will last for.
Balanced Equation:
Mg + 2HCl = MgCl2 + H2
The balanced equation is needed to determine the mole ratio between the two reactants. From the equation we know that the equation mole ratio is 1:2. However, we need the actual mole ratio to find out the limiting reactant. The actual mole ratio is calculated below:
Moles of 5.0g of Mg = 5.0/24.305 = 0.21 moles
Moles of HCl in 10ml of 6.0M = 6/1 x 0.01L = 0.06 moles Actual Mole Ratio (AMR) = 0.21/0.06 = 3.5
Equation mole Ratio (EMR) = 1/2 = 0.5
If AMR > EMR then the 2nd reactant, i.e. HCl is the limiting reactant and Magnesium is in excess.
Hypothesis: Hydrochloric acid will be the limiting react at stages 5 and 6 and Magnesium will be the limiting reactant at stage 7.
Procedure:
We were given two magnesium ribbons and a beaker of 6M HCl to carry out our experiment. Firstly, we recorded the mass of one of the Mg ribbon and wrote down any observations in our table. Then, we put the piece of magnesium ribbon into a clean test tube and added 10 drops of 6M HCl. We wrote down our observations while the reaction took place. We waited for the reaction to stop and added another 10 drops and watched the reaction for any significant changes. Then we added one drop at a time until the Magnesium had fully reacted. The table below contains all our observations: Substance (s) | Observations | Mg | * Silver grey * Around 5 cm long * Striped | Mg + 10 drops HCl | * Bubbles were produced * Reaction lasted for 10-15 seconds * Solvent has a chalky colour * Mg strip is a bit paler | Mg + 20

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