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A sample of 2.51 g of hydrated sodium carbonate (washing soda) crystals, Na2CO3·xH2O, was dissolved in deionised water and the solution made up to 250 cm³ in a volumetric flask - Leaving Cert Chemistry - Question 2 - 2001
Question 2
A sample of 2.51 g of hydrated sodium carbonate (washing soda) crystals, Na2CO3·xH2O, was dissolved in deionised water and the solution made up to 250 cm³ in a volum... show full transcript
Worked Solution & Example Answer:A sample of 2.51 g of hydrated sodium carbonate (washing soda) crystals, Na2CO3·xH2O, was dissolved in deionised water and the solution made up to 250 cm³ in a volumetric flask - Leaving Cert Chemistry - Question 2 - 2001
Step 1
Describe the correct procedure for weighing and making up the solution from hydrated sodium carbonate crystals.
Answer
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Weighing the Crystals:
- Use a suitable container, like a weighing boat or a beaker, to measure the mass of the hydrated sodium carbonate.
- Tare the balance before adding the crystal sample to ensure accuracy.
- Record the mass of the sodium carbonate.
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Making up the Solution:
- Transfer the weighed crystals into a clean volumetric flask.
- Rinse the container with deionised water to ensure all of the carbonate is transferred and add those washings to the flask.
- Fill the flask with deionised water up to the mark (250 cm³) ensuring the bottom of the meniscus is level with the graduation.
- Invert and mix the flask to ensure the solution is uniform.
Step 2
Step 3
Describe the correct procedure for washing the pipette and using it to measure the sodium carbonate solution.
Answer
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Washing the Pipette:
- Rinse the pipette with deionised water followed by rinsing with the sodium carbonate solution. This ensures that no contaminants affect the measurement.
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Using the Pipette:
- Use a pipette filler to suck the solution into the pipette, filling it above the mark.
- Let the solution drain until the bottom of the meniscus is accurate with the mark.
- Transfer the solution into the appropriate receptacle without allowing any residue to remain in the pipette.
Step 4
Assuming that the burette has been properly rinsed, state three precautions that should be taken when carrying out an accurate measurement.
Answer
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Remove Air Bubbles: Ensure there are no air bubbles in the burette tip before starting the titration; this could affect the volume dispensed.
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Read at Eye Level: Always read the burette graduation at eye level to avoid parallax errors.
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Avoid Over-Titrating: Make sure to swirl the solution in the flask while titrating and stop the addition just as the colour change occurs to avoid overshooting the endpoint.
Step 5
Calculate the concentration of the sodium carbonate in the washing solution in mol dm⁻³.
Answer
We start with the reaction:
ightarrow 2 ext{NaCl} + ext{H}_2 ext{O} + ext{CO}_2$$ From the mean titration value of 20.0 cm³ of 0.10 mol dm⁻³ HCl used: 1. Calculate moles of HCl: $$ ext{Moles} = ext{Concentration} imes ext{Volume}$$ $$= 0.10 imes rac{20.0}{1000} = 0.002 ext{ moles of HCl}$$ 2. From stoichiometry, 1 mole of Na2CO3 reacts with 2 moles of HCl, hence: $$ ext{Moles of Na}_2 ext{CO}_3 = rac{0.002}{2} = 0.001 ext{ moles}$$ 3. Since this was in 25.0 cm³ solution: $$ ext{Concentration} = rac{0.001}{ rac{25.0}{1000}} = 0.04 ext{ mol dm}^{-3}$$Step 6
Calculate the value of x, the degree of hydration, of the crystals.
Answer
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We know:
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From the previous calculation, moles of Na2CO3 in 2.51 g:
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Moles calculated from mass:
ightarrow 0.0236 ext{ moles}$$
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Using the relation in hydrated form:
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Set equal to total mass: rac{2.51}{106 + 18x} ext{ leads to } x = 7.4 ext{, approximating to } 8
Step 7
Sodium carbonate crystals, Na2CO3·xH2O, is not a primary standard but anhydrous sodium carbonate, Na2CO3, may be used as a primary standard. Why is this the case?
Answer
Anhydrous sodium carbonate, Na2CO3, is preferred as a primary standard because it is pure and has a known and constant composition. Hydrated sodium carbonate can lose water when exposed to air, and its composition may vary, leading to inaccuracies in titrations. Therefore, Na2CO3 is more reliable for standardization.