Group 2: Alkaline earth metals
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- When aqueous ammonia is added to a cation in solution, e.g. magnesium, is it the hydroxide, magnesium hydroxide that is produced, or is it the amine, magnesium amine that is produced which is responsible for the white precipitate formed? Basically, what I generally want to know is what makes the precipitate form when aqueous ammonia is added to a cation? 230310
- Hi, I am doing an investigation on hard water and ways to soften it. This is the ‘recipe’ I have come up with to make a standard solution of Ca2+ ions at 2.8 x10-3 mol dm-3. Have I calculated correctly how much calcium carbonate I need to weigh out - how do I figure out if this is correct? Standard solution of calcium ions at concentration 2.8 x 10-3 mol dm-3: 1. Weigh out 1.40 g of calcium carbonate onto a watch glass. 2. Transfer this amount into a 2 dm3 volumetric flask. 3. Rinse the watch glass with distilled water into the volumetric flask. 4. Add approximately 1400 cm3 of distilled water into the volumetric flask. 5. Next add 4cm3 of HCl to the volumetric flask 6. Swirl the contents until all of the calcium carbonate has dissolved 7. Next add dilute NaOH until the pH of the solution reaches 7-8 (check using a pH meter) 8. Then, add distilled water to just below the line on the volumetric flask. 9. Add the final drops of water with a pipette to ensure the bottom of the meniscus is on the line. 10. Put a lid on the flask and invert it several times to make sure the solution is thoroughly mixed. Also I was using the following titration recipe to calculate the concentration once I have softened the standard solutions. 1. Using a 25.0 cm3 pipette, transfer 100.0 cm3 of the calcium ion solution into a conical flask. 2. Add around 10cm3 of NaOH (until pH reaches 12) 3. Add 5 drops of Calgon indicator. 4. Place 0.010 mol dm-3 solution of disodium EDTA in a burette 5. Titrate this solution into the sample of calcium and magnesium ions until there is a colour change at the end point from red to blue. 6. Repeat this titration at least 3 times or until you have 3 readings within 0.1 of each other On step 2 why is it necessary to add NaOH - is it because the Calcon indicator will only work at pH 12? Also, I was going to try using Patton-Reeder indicator so I can see which indicator works best. Would I also need to add NaOH to this one, or pH 10 buffer solution, and why? I am very confused about why NaOH/buffer solution is added and any help would be greatly appreciated. 200210
- I’m doing my investigation on water hardness using standard solutions of calcium, magnesium, and calcium and magnesium ions. To find the concentration of just the calcium ions in the calcium and magnesium solution I intend to do the following: 1. Using a 50 cm3 pipette, transfer 100.0 cm3 of the calcium and magnesium solution into a conical flask. 2. Add around 10 cm3 of NaOH (until pH reaches 12). 3. Add 5 drops of Calcon indicator. 4. Place 0.010 mol dm-3 solution of disodium EDTA in a burette. 5. Titrate this solution into the sample of calcium and magnesium ions until there is a colour change at the end-point from red to blue. 6. Repeat this titration at least 3 times or until you have 3 readings within 0.1 cm3 of each other. But I have a few queries. Firstly, on step 2, I am adding the NaOH to precipitate out the magnesium ions. But why does this happen and why don’t the calcium ions precipitate out, I know that Mg(OH)2 is less soluble than Ca(OH)2, but why does it have to be at pH 12? Also, before I do step 5 should I filter off the magnesium hydroxide precipitate. To do this is it better to use a normal filter or do I need to use a Buchner filter? Also, whilst filtering is it OK to add distilled water to the solution or would that affect the end-point of the titration? Lastly, I was planning to do exactly the same steps as above to find the concentration of the calcium ions in the standard solution of just the calcium ions. However, as there will be no magnesium in this solution do I need to add the NaOH? Or is it essential so the indicator can work. If I don’t add the NaOH will I need to add the pH 10 buffer solution instead, like I intend to do in the magnesium titration with Solochrome black indicator, or again is this only so the indicator can work? 270110
updated: 20 September 2003
