The objective is to determine the percentage of calcium carbonate (CaCO3) in toothpaste by performing back titration and demonstrating quantitative transfer of solids and liquids. Firstly pipette 10.0ml of 0.1600M of HCl into a conical flask, using analytical balance measure 0.1-0.2g of toothpaste, and mix it with the HCl with 10.0ml of deionised to wash it down. Next, insert a funnel and heat the flask over a hot plate until the reaction is completed, allow it to cool to room temperature before adding 1 to 2 drops of methyl orange indicator and back titrate excess HCL with 0.0800M NaOH. Repeat experiment 2 times. Back titration results indicate at 16.1ml solution is red, at 16.15 ml its red-orange, at 16.20ml its orange, at 16.25ml its orange yellow and at 16.30ml its yellow. At 16.20ml, the indicator is orange indicating the point of neutralisation. The findings show the amount of CaCO3 present in the toothpaste sample is 21.5%, which is between 18%-22% in the acceptable range, therefore, the experiment is a success.

Using back titration to determine calcium carbonate (CaCO3) content. The substance being analysed in this case is CaCO3 which is insoluble and does not dissolve in water. Therefore, using titration would not yield a result. Hence it is preferable to add more acid (or base) to dissolve it first then back titrate the unreacted acid (or base) with a base (or acid).
Toothpaste contains about 18%-22% of calcium carbonate, humectant systems, fluoride, flavouring and detergents. Calcium carbonate is a white powder with low solubility in water, decompose to produce carbon dioxide when heated and liberates carbon dioxide as a by-product. It is used as a material for construction as an ingredient in cement. Medicinal purposes like manufacturing antacids, calcium supplements and manufacture of paper, plastics and paints, etc. (ByJu, 2018)

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Sodium Hydroxide is a white solid, popularly known as a caustic soda. It is soluble in polar solvents but insoluble in non-polar solvents. Used to manufacture paper, soaps and detergents. It is also used for degreasing metals and remove sulphurous impurities from crude oil via caustic washing. (ByJu, 2018)

Hydrochloric acid is a strong monoprotic acid it is colourless and has a sour with a pungent odour. Industrial uses include manufacture of plastics, dyes and fertilizers, also in the textile and rubber industries, cleaning products and gelatine. (Softschool, 2018)

Methyl orange is used as an indicator for titration changing colour at the pH3.4-4.1of an acid, as the solution becomes less acidic it changes from to red to orange and to yellow, and vice versa for a solution becoming more acidic.

The rationale of conducting the experiment is to determine the concentration of calcium carbonate (CaCO3) in toothpaste. As CaCO3 acts as an abrasive, too little of it reduces the effect of cleaning the teeth and removing plaque. While too much of it is not good as well, because it can harm the teeth sensitivity by weakening enamel, discolouration and cause teeth to become sensitive. Therefore, conducting the experiment to determine acceptable amount is more effective.

Back titration is the reverse direction of a normal titration and is used to determine the concentration of an analyte by reacting it with a known amount of excess reagent. The excess reagent, hydrochloric acid (HCl) is titrated with another reagent, sodium hydroxide (NaOH). The results show how much of the excess reagent is used in the first titration and the concentration of the analyte can therefore be calculated. It is used in an acid-base titration when the substance is insoluble, and the end point of titration is hard to determine.

NaOH + HCL (excess) ? NaCl + H2O

Carbon dioxide is an acidic oxide that can affect the pH of the solution hence by heating up the solution it removes the carbon dioxide via evaporation, also speeding up the reaction at the same time

CaCO3 + 2HCl ? CaCl2 + CO2 +H2O

Firstly, the mass of the toothpaste obtained was recorded down, followed by the volume of sodium hydroxide (NaOH) used during the titration process. By using the mole ratio, the volume of excess hydrochloric acid (HCl) was calculated out, to find the volume of HCl reacted by subtracting the original amount with the excess HCl. Using mole ratio moles of HCl reacted was determined, from the balanced equation moles of calcium carbonate (CaCO3) is therefore determined as well. The mass of CaCO3 to calculate out the percent of CaCo3 in the toothpaste.

The percentage of calcium carbonate for the experiment is 21.5% which is in the acceptable range. From the results, the higher the mass of toothpaste the higher the mass of CaCO3 present in the toothpaste, 0.1380g of toothpaste results in 0.0299g of CaCO3 and 0.1323g of toothpaste results in 0.0283g of CaCO3. Also, with a higher mass of CaCO3 the lower the volume of sodium hydroxide (NaOH) required to neutralise the acid-toothpaste solution,0.1380g of toothpaste requires 13.50mL of NaOH to neutralise while 0.1323g of toothpaste requires 13.90mL of NaOH for it to turn orange colour as the indicator used is methyl orange.
The theoretical value should have a value close to 20%, however this is not the case as the results shows 21.5% this could be due to personal errors like self determination of the endpoint of titration, as the colour changes specifically is determined by an individual. Repeating the experiment, a couple of times could help to reduce such errors to achieve more accurate results. The percentage should not be too low which reduces the effects of removing plaque from teeth and should not be too high causing discolouration, weakening of enamel and sensitivity.

From the experiment findings it is deduced that the average percentage of calcium carbonate (CaCO3) is 21.5% which is in the acceptable range in toothpaste. During the titration, at 16.10mL of sodium hydroxide (NaOH) with the hydrochloric acid (HCl) the indicator is red, at 16.20mL it is orange and finally at 16.30mL it is yellow. Although the experiment requires us to repeat twice, I repeated thrice due to a error during the titration phrase of adding to much NaOH.Which caused the calculation of mass of CaCO3 to be way above the acceptable percentage hence it was redone to achieve a more accurate results. However overall the experiment was a success as the theoretical results coincides with the experimental results gotten, and the error that caused the results to deviate is solved by redoing the experiment.


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