The calcium in a 150 mL sample of water from the Ogeechee River at a location near Savannah was determined by precipitating Ca2+ as CaC2O4.H2O (FW=146.115 g/mol). The precipitate was collected by filtration, washed and then ignited [turning the CaC2O4.H2O into CaO (FW=56.079 g/mol)] in a crucible that had an initial mass of 26.4092 g. The mass of the crucible plus CaO was 26.7134 g). What is the concentration of Ca (40.078 g/mol) in the river water in units of part per thousand (ppt) (by mass), assuming the river water has the same density as pure water?

Question

The calcium in a 150 mL sample of water from the Ogeechee River at a location near Savannah was determined by precipitating Ca2+ as CaC2O4.H2O (FW=146.115 g/mol). The precipitate was collected by filtration, washed and then ignited [turning the CaC2O4.H2O into CaO (FW=56.079 g/mol)] in a crucible that had an initial mass of 26.4092 g. The mass of the crucible plus CaO was 26.7134 g). What is the concentration of Ca (40.078 g/mol) in the river water in units of part per thousand (ppt) (by mass), assuming the river water has the same density as pure water?

Answer 1

26.7134 = mass xble + mass CaO

-26.4092 = mass xble
-----------
00.3042 = mass CaO

Mass Ca = 0.3042 x (mass Ca/mass CaO) = 0.3042 x (40.078/56.079) = about 0.218 g Ca.

% Ca = (mass Ca/mass sample)*100 =
(0.218g/150g)*100 = 0.145% which actually is 0.145 pph (parts per 100). So just multiply top and bottom to get parts per thousand (1.45 ppt). You can convert to ppm easily from this same set up.

Answer 2

To find the concentration of calcium (Ca) in the river water in units of parts per thousand (ppt), we need to calculate the mass of Ca in the sample and compare it to the total mass of the sample. Here's how we can do that step by step:

1. Determine the mass of the CaO formed:
Mass of crucible + CaO - Mass of crucible = Mass of CaO
26.7134 g - 26.4092 g = 0.3042 g

2. Calculate the moles of CaO formed:
Moles of CaO = Mass of CaO / Molar mass of CaO
Moles of CaO = 0.3042 g / 56.079 g/mol ≈ 0.00543 mol

3. Since the reaction is 1:1 between CaO and Ca, the moles of Ca are also equal to 0.00543 mol.

4. Calculate the mass of Ca in the sample:
Mass of Ca = Moles of Ca x Molar mass of Ca
Mass of Ca = 0.00543 mol x 40.078 g/mol ≈ 0.217 g

5. The concentration of Ca in the river water is given in parts per thousand (ppt), which is equivalent to grams of solute per thousand grams of solution. Since the density of water is approximately 1 g/mL, the mass of the river water sample is equal to its volume.

6. Calculate the concentration of Ca in ppt:
Concentration of Ca = (Mass of Ca / Mass of water sample) x 1000
Volume of water sample = 150 mL = 150 g
Concentration of Ca = (0.217 g / 150 g) x 1000 ≈ 1.45 ppt

Therefore, the concentration of Ca in the river water near Savannah is approximately 1.45 parts per thousand (ppt) by mass.