5.0g of magnesium metal reacted with 1000cm3 of 4.5M of HCl solution to produce hydrogen gas. Calculate the amount of HCl left and the number of hydrogen molecules produced
This is a limiting reagent problem. You know that because amounts are given for both reactants. Here is a worked example of a limiting reagent problem. Just follow the steps.
http://www.jiskha.com/science/chemistry/limiting_reagent_problem.html
To calculate the amount of HCl left and the number of hydrogen molecules produced, we can follow these steps:
Step 1: Convert the given quantities to the appropriate units for calculations.
The given mass of magnesium is 5.0g, and the volume of the HCl solution is 1000cm3. To perform calculations, we need to convert these values to moles.
The molar mass of magnesium (Mg) is 24.31 g/mol.
Number of moles of Mg = mass / molar mass
= 5.0g / 24.31 g/mol
≈ 0.206 mol
Step 2: Determine the stoichiometry of the reaction.
Balanced equation: Mg + 2HCl → MgCl2 + H2
From the equation, we can see that one mole of magnesium reacts with two moles of HCl to produce one mole of hydrogen gas.
Step 3: Determine the number of moles of HCl reacted.
Number of moles of HCl = concentration (M) x volume (L)
Given that the concentration of HCl is 4.5M and the volume is 1000cm3 (which is 1 L after conversion), we can calculate:
Number of moles of HCl = 4.5M x 1L
= 4.5 mol
Step 4: Determine the limiting reactant.
To find the limiting reactant, we compare the number of moles of each reactant with the stoichiometric ratio.
The stoichiometric ratio (Mg:HCl) is 1:2, which means for every 1 mole of magnesium, we need 2 moles of HCl. Since we have 0.206 mol of Mg and 4.5 mol of HCl, we can calculate:
Number of moles of HCl needed = 2 x 0.206 mol (stoichiometric ratio)
= 0.412 mol
Since we have more HCl (4.5 mol) than required (0.412 mol), magnesium is the limiting reactant.
Step 5: Calculate the amount of HCl left.
To calculate the amount of HCl left, subtract the moles of HCl reacted from the total moles of HCl initially present.
Amount of HCl left = Total moles of HCl - Moles of HCl reacted
= 4.5 mol - 0.412 mol
≈ 4.088 mol
Step 6: Calculate the number of hydrogen molecules produced.
Since we have determined that magnesium is the limiting reactant, the moles of hydrogen gas produced will be equal to the moles of magnesium used.
Number of moles of H2 = Moles of Mg used
= 0.206 mol
Finally, we can convert the number of moles of hydrogen gas to the number of molecules using Avogadro's constant (6.022 x 10^23 molecules/mol):
Number of hydrogen molecules produced = Moles of H2 x Avogadro's constant
= 0.206 mol x 6.022 x 10^23 molecules/mol
Note: I did not perform the final calculation as the value of moles of hydrogen was not provided in the question. You can substitute the given value to find the exact number of hydrogen molecules produced.