An alloy is an intimate mixture of elements, usually metals. When 76.20 g of an alloy composed only of silver and copper were added to an excess of concentrated nitric acid the following reactions took place. Brown fumes of nitrogen(IV) oxide gas were released and 22.50 g of copper(II) nitrate were formed. $$ Ag + 2HNO_3 (aq) \rightarrow 4AgNO_3 (aq) + NO_2 (g) + H_2O (l) $$ $$ Cu + 4HNO_3 (aq) \rightarrow Cu(NO_3)_2 (aq) + 2NO_2 (g) + 2H_2O (l) $$ (i) Find the mass of silver in the alloy. (ii) What was the ratio by mass of silver to copper in the alloy? (iii) What total volume of nitrogen(IV) oxide gas, measured at st.p., was released in these reactions?

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An alloy is an intimate mixture of elements, usually metals - Leaving Cert Chemistry - Question c - 2021

Question c

An alloy is an intimate mixture of elements, usually metals. When 76.20 g of an alloy composed only of silver and copper were added to an excess of concentrated nitr... show full transcript

Worked Solution & Example Answer:An alloy is an intimate mixture of elements, usually metals - Leaving Cert Chemistry - Question c - 2021

Step 1

Find the mass of silver in the alloy.

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Answer

To find the mass of silver in the alloy, we need to calculate the moles of copper(II) nitrate formed. Given that 22.50 g of Cu(NO3)2 were formed, we use the molar mass for Cu(NO3)2.

The molar mass of Cu(NO3)2 is: $M_{Cu(NO_3)_2} = 63.55 + 2(14.01 + 16.00 imes 3) = 187.55 ext{ g/mol}$

Calculating the number of moles of Cu(NO3)2: $ext{Moles of } Cu(NO_3)_2 = \frac{22.50 ext{ g}}{187.55 ext{ g/mol}} \approx 0.120 ext{ moles}$

From the balanced reaction, we see that 1 mole of Cu produces 1 mole of Cu(NO3)2. Thus, the moles of copper used is also approximately 0.120 moles. Since the alloy is 76.20 g in total, we can now find the mass of silver (Ag) using the moles of Ag which is left.

The total moles initially (Ag + Cu):

Let the moles of silver be x.
Total moles: x + 0.120 = total mass of Ag / molar mass of Ag

Calculating silver's mass: $76.20 - (0.120 imes 63.55 ext{ g/mol}) = 76.20 - 7.626 = 68.58 ext{ g for silver}$ .

Therefore, the mass of silver in the alloy is approximately 68.58 g.

Step 2

What was the ratio by mass of silver to copper in the alloy?

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Answer

The mass of silver is calculated as 68.58 g and the mass of copper used was: $ext{Mass of copper} = 76.20 g - 68.58 g = 7.62 g$

Now, to find the ratio by mass of silver to copper:
$ext{Mass ratio} = \frac{68.58}{7.62} \approx 9:1$

Thus, the ratio by mass of silver to copper in the alloy is 9:1.

Step 3

What total volume of nitrogen(IV) oxide gas, measured at st.p., was released in these reactions?

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Answer

From the moles calculated, each mole of silver produces 1 mole of NO2 and copper produces 2 moles.
Total moles of NO2 produced:

For silver: 0.120 moles yield 0.120 moles NO2
For copper: 0.120 moles yield 0.240 moles NO2

Total NO2 produced: $0.120 + 0.240 = 0.360 ext{ moles}$

At standard temperature and pressure (s.t.p), one mole of gas occupies 22,400 cm³.
Therefore, to find the volume of NO2: $ext{Volume} = 0.360 ext{ moles} \times 22,400 ext{ cm}^3 = 8064 ext{ cm}^3$

Thus, the total volume of nitrogen(IV) oxide gas released in these reactions is approximately 8064 cm³.

An alloy is an intimate mixture of elements, usually metals - Leaving Cert Chemistry - Question c - 2021

Worked Solution & Example Answer:An alloy is an intimate mixture of elements, usually metals - Leaving Cert Chemistry - Question c - 2021

Find the mass of silver in the alloy.

What was the ratio by mass of silver to copper in the alloy?

What total volume of nitrogen(IV) oxide gas, measured at st.p., was released in these reactions?

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