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. $\text{Ag} + 2 \text{HNO}_3(\text{aq}) \rightarrow \text{AgNO}_3(\text{aq}) + \text{NO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})$ $\text{Cu} + 4 \text{HNO}_3(\text{aq}) \rightarrow \text{Cu(NO}_3\text{)}_2(\text{aq}) + 2 \text{NO}_2(\text{g}) + 2 \text{H}_2\text{O}(\text{l})$ (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 s.t.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 start by calculating the moles of copper(II) nitrate produced:

The molar mass of copper(II) nitrate, Cu(NO₃)₂, is 187.5 g/mol.
The moles of Cu(NO₃)₂ produced:
$\text{Moles of Cu(NO}_3\text{)}_2 = \frac{22.50 g}{187.5 g/mol} = 0.120 \text{ moles}$
According to the reaction, 1 mole of copper reacts to produce 1 mole of Cu(NO₃)₂. Therefore, the moles of copper in the alloy are 0.120 moles.
The mass of copper in the alloy:
$\text{Mass of Cu} = \text{Moles of Cu} \times \text{Molar Mass of Cu} = 0.120 \text{ moles} \times 63.55 g/mol = 7.626 g$
Finally, the mass of silver in the alloy:
$76.20 g - 7.626 g = 68.58 g \text{ Ag}$

Step 2

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

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Answer

To find the mass ratio of silver to copper:

We have:
- Mass of silver = 68.58 g
- Mass of copper = 7.626 g
The ratio of silver to copper:
$\text{Ratio} = \frac{68.58 g}{7.626 g} \approx 9:1$ This ratio can also be expressed as 9:1 for every 1 part copper to 9 parts silver.

Step 3

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

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Answer

To find the total volume of nitrogen(IV) oxide, we consider the contributions from both reactions:

From the silver reaction, 0.120 moles of Cu(NO₃)₂ produces 0.120 moles of NO₂ gas.
For the copper reaction,:
- Each mole of copper produces 2 moles of NO₂.
- Therefore, the moles of NO₂ from 0.120 moles of copper are: $0.120 \text{ moles Cu} \times 2 = 0.240 \text{ moles NO}_2$
The total moles of NO₂ released:
$0.120 + 0.240 = 0.360 \text{ moles NO}_2$
At standard temperature and pressure (s.t.p), 1 mole of gas occupies 22.4 liters. The total volume: $\text{Volume} = 0.360 \text{ moles} \times 22.4 \text{ liters/mole} = 8.064 \text{ liters}$ Thus, the total volume of nitrogen(IV) oxide gas released is approximately 8.064 liters.

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 s.t.p., was released in these reactions?

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