Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate, CaCO3. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. A sample of chalk was analyzed for its calcium carbonate content. The chalk was ground to a fine
powder and a 0.2996 g sample was placed in a 250 mL Erlenmeyer flask. Exactly 50 mL of a 0.1525 molar solution of hydrochloric acid was pipeted into the flask, whereupon the chalk dissolved with considerable foaming as the following reaction occurred.
CaCO3 (s) + 2 HCl �¨ CaCl2 (aq) + H2O + CO2 (g) After the foaming stopped, the result was a homogeneous solution that was acid, because excess HCl was added. A drop of phenolphthalein was added and the excess HCl was titrated with a 0.1122 molar solution of sodium hydroxide. The titration required 39.02 mL NaOH solution to neutralize the excess acid. What was the weight percent of calcium carbonate in the chalk?
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To find the weight percent of calcium carbonate in the chalk, we need to use the data provided and perform some calculations. Here's how you can do it:
Step 1: Calculate the number of moles of hydrochloric acid used.
Using the molarity and volume of the hydrochloric acid solution, we can calculate the number of moles of HCl used in the reaction.
Moles of HCl = Molarity of HCl x Volume of HCl used
Moles of HCl = 0.1525 mol/L x 0.050 L
Moles of HCl = 0.007625 mol
Step 2: Determine the stoichiometry of the reaction.
From the balanced chemical equation, we can see that 1 mole of CaCO3 reacts with 2 moles of HCl. Therefore, the moles of CaCO3 reacted would be twice the moles of HCl used.
Moles of CaCO3 = 2 x Moles of HCl
Moles of CaCO3 = 2 x 0.007625 mol
Moles of CaCO3 = 0.01525 mol
Step 3: Calculate the molar mass of CaCO3.
The molar mass of CaCO3 is the sum of the atomic masses of calcium, carbon, and three oxygen atoms.
Molar mass of CaCO3 = (molar mass of Ca) + (molar mass of C) + (3 x molar mass of O)
Molar mass of CaCO3 = (40.08 g/mol) + (12.01 g/mol) + (3 x 16.00 g/mol)
Molar mass of CaCO3 = 100.09 g/mol
Step 4: Calculate the mass of calcium carbonate in the sample.
The mass of calcium carbonate can be determined using the moles of CaCO3 and its molar mass.
Mass of CaCO3 = Moles of CaCO3 x Molar mass of CaCO3
Mass of CaCO3 = 0.01525 mol x 100.09 g/mol
Mass of CaCO3 = 1.525 g
Step 5: Calculate the weight percent of calcium carbonate.
The weight percent can be calculated by dividing the mass of calcium carbonate by the total mass of the chalk sample and multiplying by 100.
Weight percent of CaCO3 = (Mass of CaCO3 / Mass of chalk) x 100
Weight percent of CaCO3 = (1.525 g / 0.2996 g) x 100
Weight percent of CaCO3 = 509.33%
Therefore, the weight percent of calcium carbonate in the chalk sample is approximately 509.33%. This result seems unusually high, so please double-check the provided data and calculations to ensure accuracy.
To determine the weight percent of calcium carbonate in the chalk, we need to calculate the number of moles of calcium carbonate (CaCO3) reacted with hydrochloric acid (HCl) and then use that information to find the weight percent.
Here's the step-by-step process to find the weight percent:
1. Calculate the number of moles of hydrochloric acid (HCl) used:
Using the given molarity (0.1525 M) and volume (50 mL), we can calculate the moles of HCl as follows:
Moles of HCl = Molarity × Volume (in L)
Moles of HCl = 0.1525 mol/L × (50 mL / 1000 mL/L)
Moles of HCl = 0.007625 mol
2. From the balanced chemical equation, we can see that 1 mole of calcium carbonate (CaCO3) reacts with 2 moles of hydrochloric acid (HCl).
So, the number of moles of calcium carbonate reacted will be half of the moles of HCl used:
Moles of CaCO3 = (1/2) × Moles of HCl
Moles of CaCO3 = (1/2) × 0.007625 mol
Moles of CaCO3 = 0.0038125 mol
3. Calculate the molecular weight of calcium carbonate (CaCO3):
The molecular weight of calcium carbonate is calculated by adding the atomic weights of calcium (Ca), carbon (C), and three oxygen (O) atoms:
Molecular weight of CaCO3 = (1 × Atomic weight of Ca) + (1 × Atomic weight of C) + (3 × Atomic weight of O)
Molecular weight of CaCO3 = (1 × 40.08 g/mol) + (1 × 12.01 g/mol) + (3 × 16.00 g/mol)
Molecular weight of CaCO3 = 100.09 g/mol
4. Calculate the weight of calcium carbonate used:
Weight of CaCO3 = Moles of CaCO3 × Molecular weight of CaCO3
Weight of CaCO3 = 0.0038125 mol × 100.09 g/mol
Weight of CaCO3 = 0.3816 g
5. Calculate the weight percent of calcium carbonate in the chalk:
Weight percent of CaCO3 = (Weight of CaCO3 / Weight of Chalk) × 100
We are given that the weight of chalk used is 0.2996 g (given in the question).
Weight percent of CaCO3 = (0.3816 g / 0.2996 g) × 100
Weight percent of CaCO3 = 127.37%
Therefore, the weight percent of calcium carbonate in the chalk is approximately 127.37%.