A 100ml aliquot of a water sample containing Ca(II) and Mg(II) is titrated with 22,74mL of EDTA 0.00998 mol/L at PH 10. Another aliquot of 100ml of the same sample is treated with NaOH to precipitate Mg(OH)2 and is then titrated with PH 13 with 15.86 ML of the same EDTA solution. Calculate the total hardness (mg/ML CaCO3). CaCO3 = 100.09 g/mol.
I may be missing something here but I don't know why the second step of pptng the Mg(OH)2 is necessary since EDTA at pH of 10 titrates both Mg and Ca.
millimols Ca + Mg = mL x M EDTA, then mmols x molar mass CaCO3 = mg CaCO3/100 mL. Convert to mg/L.
To calculate the total hardness of the water sample in mg/mL CaCO3, we need to determine the concentration of Ca(II) and Mg(II) ions in the sample.
Let's break down the steps to find each ion's concentration:
Step 1: Calculate the number of moles of EDTA used in the titration.
For the first titration:
Volume of EDTA used = 22.74 mL
Concentration of EDTA = 0.00998 mol/L
Number of moles of EDTA = Volume x Concentration
Number of moles of EDTA = 22.74 mL * 0.00998 mol/L
Step 2: Calculate the number of moles of Ca(II) and Mg(II) ions in the water sample.
Since we are titrating with EDTA, which forms 1:1 complex with Ca(II) and Mg(II), the number of moles of Ca(II) and Mg(II) ions will be equal to the number of moles of EDTA used.
Number of moles of Ca(II) and Mg(II) ions = Number of moles of EDTA
Step 3: Calculate the concentration of Ca(II) and Mg(II) ions in the water sample.
For the first titration, we used a 100 mL aliquot of the water sample:
Volume of water sample = 100 mL = 0.1 L
Concentration of Ca(II) and Mg(II) ions = Number of moles / Volume
Concentration of Ca(II) and Mg(II) ions = Number of moles of Ca(II) and Mg(II) ions / Volume of water sample
Concentration of Ca(II) and Mg(II) ions = Number of moles of EDTA / Volume of water sample
Step 4: Repeat steps 1-3 for the second titration to find the concentration of Ca(II) ions.
For the second titration:
Volume of EDTA used = 15.86 mL
Number of moles of EDTA = 15.86 mL * 0.00998 mol/L
Since we are precipitating Mg(OH)2, only the Ca(II) ions will react with EDTA in the second titration.
Number of moles of Ca(II) ions = Number of moles of EDTA
Volume of water sample = 100 mL = 0.1 L
Concentration of Ca(II) ions = Number of moles of Ca(II) ions / Volume of water sample
Step 5: Calculate the total hardness of the water sample.
Total hardness = Concentration of Ca(II) ions + Concentration of Mg(II) ions
Note: The units of the concentration of Ca(II) and Mg(II) ions will be mol/L. To convert the total hardness to mg/mL CaCO3, you need to convert mol/L to mg/ML using the molar mass of CaCO3.
Total hardness (mg/ML CaCO3) = Total hardness (mol/L) x Molar mass of CaCO3 (mg/mol)
Substitute the values obtained in the calculations and calculate the total hardness.