A .500 g sample of impure CaO (a solid) is added to 50.0 mL of 0.100 M HCl. The excess HCl is titrated by 5.00 mL of .125 M NaOH. What is the percentage of CaO in the sample?
CaO + 2HCl ==> CaCl2 + H2O
mols HCl to start = M x L = ?
mols HCl excess = M x L = 0.125 M x 0.005 L = ?
Subtract. The difference is mols HCl used by the CaO.
mols CaO = 1/2 mols HCl (the coefficients tell you that).
g CaO = mols x molar mass
%CaO = (mass CaO/ mass sample)*100 = ?
To find the percentage of CaO in the sample, we need to determine the amount of CaO reacted with HCl and compare it to the original mass of the sample.
Let's break down the problem step-by-step:
Step 1: Calculate the moles of HCl used.
To find the moles of HCl used, we can use the concentration of HCl and the volume used:
moles of HCl = concentration of HCl (in moles/L) × volume of HCl (in L)
Given:
Concentration of HCl = 0.100 M
Volume of HCl = 50.0 mL = 50.0 mL × (1 L / 1000 mL) = 0.0500 L
moles of HCl = 0.100 M × 0.0500 L = 0.00500 moles
Step 2: Determine the moles of CaO reacted.
From the balanced chemical equation between CaO and HCl, we know that 1 mole of CaO reacts with 2 moles of HCl:
CaO + 2HCl -> CaCl2 + H2O
Therefore, moles of CaO reacted = 0.00500 moles / 2 = 0.00250 moles
Step 3: Calculate the mass of CaO reacted.
To find the mass of CaO reacted, we need to use the molar mass of CaO, which is 56.08 g/mol:
mass of CaO reacted = moles of CaO reacted × molar mass of CaO
mass of CaO reacted = 0.00250 moles × 56.08 g/mol = 0.140 g
Step 4: Calculate the percentage of CaO in the sample.
The percentage of CaO in the sample is the mass of CaO reacted divided by the mass of the sample, multiplied by 100%:
percentage of CaO = (mass of CaO / mass of sample) × 100%
mass of sample = 0.500 g (given)
percentage of CaO = (0.140 g / 0.500 g) × 100% = 28.0%
Therefore, the percentage of CaO in the sample is 28.0%.