Question

Once we know the amount of EDTApresent when the color change occurs,we can determine the concentration ofcalcium present in the tap water byconverting the volume of EDTA toA. grams and then multiple that number by 40.01 grams.B. moles and then take that number and divide it by the initialvolume of tap water in liters.C. moles and then multiply that number by 2 and divide it bythe molar mass of calcium.

Answer 1

Step 1 - Understanding the reaction between Calcium and EDTA

EDTA is a molecule that always reacts in an equimolar fashion with any metal. Therefore, we always need one mole of EDTA for one mole of the metal.

We can represent its reaction with Ca(2+) in a simplified manner as:

`\text{EDTA}_{}+Ca^(2+)\to EDTA-Ca\text{ complex}`

Since the reaction is equimolar, the quantity spent in moles of EDTA is exactly the same as the quantity of moles of Ca(2+) in the tap water sample.

Step 2 - How can we calculate the concentration of Ca(2+) in tap water?

Once we know the amount of EDTA present when the color change occurs,

we can determine the concentration of calcium present in the tap water by

converting the volume of EDTA to...?

Well, first of all, let's remember the definition of concentration:

`\lbrack Ca^(2+)\rbrack=\frac{n\text{ (mol)}}{V\text{ (L)}}`

Therefore, to obtain the concentration of Ca(2+) in tap water, we just need to divide the number of moles of Ca(2+) in the sample by its volume.

Since the number of moles of Calcium can be obtained directly by the number of moles of EDTA (see step 1), the correct way to proceed is converting the volume of EDTA to moles and then take that number and divide it by the initial volume of tap water in liters.

The correct alternative is thus item b).