SSCE VCE Chemistry Families of Organic Compounds: In order to help prevent tooth d

In order to help prevent tooth decay, fluoride ions at a level of 0.9 mg L-1 of F- are added to Melbourne’s public water supplies - VCE - SSCE Chemistry - Question 4 - 2004 - Paper 1

Question 4

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In order to help prevent tooth decay, fluoride ions at a level of 0.9 mg L-1 of F- are added to Melbourne’s public water supplies. The fluoride... show full transcript

Worked Solution & Example Answer:In order to help prevent tooth decay, fluoride ions at a level of 0.9 mg L-1 of F- are added to Melbourne’s public water supplies - VCE - SSCE Chemistry - Question 4 - 2004 - Paper 1

Step 1

i. Calculate the mass of sodium fluoride in mg that must be present in one litre of water to produce a concentration of fluoride ions of 0.90 mg L-1.

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Answer

To find the mass of sodium fluoride (NaF) needed for a 0.90 mg L-1 concentration of fluoride ions (F-), we use the molar mass of NaF.

Determine the molar mass of NaF:
- Molar mass of Na = 22.99 g/mol
- Molar mass of F = 19.00 g/mol
- Molar mass of NaF = 22.99 + 19.00 = 41.99 g/mol
Calculate the mass of NaF needed for 0.90 mg of F-:
The fluoride ion (F-) is produced from NaF in a 1:1 ratio.
- Thus, 0.90 mg of F- corresponds to 0.90 mg of NaF.
- Since there are 1000 mg in 1 g, the mass in grams is 0.90 mg = 0.00090 g.
Convert this to mg:
- Therefore, the mass of sodium fluoride required is 0.90 mg.

Step 2

ii. What mass of sodium fluoride, in kilogram, must be added to a 750 ML reservoir (1 ML = 106 L) to produce a concentration of fluoride ions of 0.90 mg L-1?

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Answer

Calculate the total volume in liters:
- 750 ML = 750 × 10-3 L = 0.750 L.
Determine total mass of fluoride ions required:
- Concentration required = 0.90 mg L-1
- Total mass of F-:
Mass = Concentration × Volume = 0.90 mg/L × 0.750 L = 0.675 mg
Calculate the mass of NaF needed:
NaF to F- ratio = 1:1.
Thus, 0.675 mg of NaF is required, which is equivalent to 0.000675 g = 0.000000675 kg.

Step 3

iii. Calculate the number of fluoride ions swallowed by a person who drank one litre of water from the reservoir.

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Answer

Determine the amount of fluoride ions in 1 L of water:
- From previous calculation, 1 L contains 0.90 mg of F-.
Convert mg to grams:
- 0.90 mg = 0.00090 g.
Determine the number of moles of fluoride ions:
- Molar mass of F = 19.00 g/mol.
- Moles of F = mass/molar mass = 0.00090 g / 19.00 g/mol = 4.736 × 10-5 mol.
Calculate the number of fluoride ions:
- Using Avogadro's number (6.022 × 1023 ions/mol):
Number of ions = moles × Avogadro's number = 4.736 × 10-5 mol × 6.022 × 1023 ions/mol ≈ 2.85 × 1019 ions.

In order to help prevent tooth decay, fluoride ions at a level of 0.9 mg L<sup>-1</sup> of F<sup>-</sup> are added to Melbourne’s public water supplies - VCE - SSCE Chemistry - Question 4 - 2004 - Paper 1

Worked Solution & Example Answer:In order to help prevent tooth decay, fluoride ions at a level of 0.9 mg L<sup>-1</sup> of F<sup>-</sup> are added to Melbourne’s public water supplies - VCE - SSCE Chemistry - Question 4 - 2004 - Paper 1

i. Calculate the mass of sodium fluoride in mg that must be present in one litre of water to produce a concentration of fluoride ions of 0.90 mg L<sup>-1</sup>.

ii. What mass of sodium fluoride, in kilogram, must be added to a 750 ML reservoir (1 ML = 10<sup>6</sup> L) to produce a concentration of fluoride ions of 0.90 mg L<sup>-1</sup>?

iii. Calculate the number of fluoride ions swallowed by a person who drank one litre of water from the reservoir.

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