Gravimetric Analysis Revision Notes for HSC SSCE Chemistry

Gravimetric Analysis

What is gravimetric analysis?

Gravimetric analysis is a method used in chemistry to find out how much of each substance is present in a mixture by measuring their masses. The word "gravimetric" comes from the idea of measuring by weight or mass. Essentially, it's analysis by mass.

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Gravimetric analysis is a quantitative analytical technique - it tells you not just what's present in a mixture, but precisely how much of each component you have.

When you perform gravimetric analysis, you're determining:

The mass of each component in a sample
The percentage composition of the mixture (what proportion of the total is each substance)

This is different from just identifying what substances are present - gravimetric analysis tells you how much of each substance you have.

Why do we use gravimetric analysis?

Understanding the exact composition of mixtures is important in many real-world situations. Here are some key reasons why scientists and industries use gravimetric analysis:

Mining and geology: When geologists discover a new mineral deposit, they need to know if it contains enough of the valuable compound to make mining it worthwhile. For example, if a deposit only contains 5% of the desired mineral, it might not be economically viable to extract it. But if it contains 60%, it could be very profitable.

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Real-world application: Mining companies use gravimetric analysis to perform cost-benefit analyses before committing millions of dollars to extraction operations. The percentage composition directly determines whether a mining operation will be profitable.

Quality control in manufacturing: Companies need to check whether their products have the correct composition. They might also want to compare their product to a competitor's product to see if the compositions match.

Product labelling: Many commercial products display their chemical composition on their labels, either as percentages or masses of different components. This information comes from gravimetric analysis.

How does gravimetric analysis work?

To carry out a complete gravimetric analysis, you need to follow these basic steps:

Take a known mass of the mixture - Weigh your starting sample carefully and record this mass
Separate the components - Use appropriate separation techniques (like filtration, evaporation, or magnetic separation) to isolate each component
Weigh each component - Once separated and dried, measure the mass of each pure component
Calculate percentages - Use the masses to work out what percentage of the mixture each component represents

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The separation techniques you learned about earlier in this chapter (filtration, evaporation to dryness, distillation, etc.) are essential tools for gravimetric analysis. The accuracy of your results depends entirely on how well you separate the components - any contamination will affect your final percentages.

Calculating percentage composition

Once you've separated and weighed the components of a mixture, you can calculate the percentage composition using this formula:

$\text{Percentage of component} = \frac{\text{mass of component}}{\text{total mass of mixture}} \times 100$

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Worked Example: Mineral Analysis

Let's look at how geologists might analyse a mineral sample they've discovered.

The scenario: A team of geologists found a new mineral in a remote desert. The mineral was a mixture of barium sulfate and magnesium sulfate. They wanted to find out the percentage composition.

What they did:

Started with a $3.61~\text{g}$ sample of the mineral
Ground it up and added water (magnesium sulfate dissolves in water, but barium sulfate doesn't)
Filtered off the barium sulfate, dried it, and weighed it: $1.52~\text{g}$
Evaporated the water from the filtrate to recover magnesium sulfate and weighed it: $2.08~\text{g}$

Step 1: Check the total

First, add up the masses of the two components:

$\text{Total mass} = 1.52 + 2.08 = 3.60~\text{g}$

This is very close to the original $3.61~\text{g}$ (the difference is less than $0.3\%$ ). This small discrepancy is due to experimental error, and it confirms the sample contains only these two sulfates.

Step 2: Calculate percentage of barium sulfate

$\text{Percentage of barium sulfate} = \frac{1.52}{3.61} \times 100 = 42.1\%$

Step 3: Calculate percentage of magnesium sulfate

Since there are only two components, we can subtract from $100\%$ :

$\text{Percentage of magnesium sulfate} = 100 - 42.1 = 57.9\%$

The answer:

The mineral sample consists of 42.1% barium sulfate and 57.9% magnesium sulfate (both with an accuracy of about $\pm 0.3\%$ ).

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Significant Figures Matter!

Make sure your final answer has the same number of significant figures as your original data. This ensures your answer isn't claiming to be more precise than your measurements actually were.

Practical investigation: Determining percentage composition

A common laboratory activity involves analysing a mixture of salt, sand, and iron filings to determine the percentage of each component. This investigation demonstrates how to apply gravimetric analysis principles in practice.

Planning your investigation

Before starting, you need to:

Identify appropriate equipment: Think about what you'll need based on the separation methods required. For a mixture of salt, sand, and iron filings, consider how the different properties of these substances can be used to separate them.

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Planning your separation sequence:

The key is to use the unique properties of each component:

Iron filings are magnetic (sand and salt are not)
Salt dissolves in water (sand and iron filings do not)

These properties determine which separation techniques to use and in what order.

Conduct a risk assessment: Identify potential hazards (such as hot glassware during evaporation) and plan how to manage these risks safely. Always get your teacher's approval before starting.

Carrying out the analysis

The general approach involves:

Weighing the initial mixture
Using magnetic separation to remove iron filings
Adding water to dissolve the salt
Filtering to separate the sand
Evaporating the water to recover the salt
Weighing each component
Calculating the percentage of each substance

Analysing your results

After completing the separation and weighing, you need to:

Calculate the percentage of each component using the formula shown earlier
Compare your results with the known composition (if provided by your teacher)
Consider sources of error and how they might have affected your results
Think about limitations in your method that could impact accuracy

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Common sources of error:

Incomplete separation of components - some mixing between substances
Loss of material during transfer between containers
Incomplete drying of components before weighing
Salt left behind in the evaporating dish

Being aware of these potential errors helps you interpret your results accurately and suggest improvements for future investigations.

Important considerations in gravimetric analysis

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Key Factors Affecting Accuracy:

Experimental error: Small differences between your calculated total mass and the original sample mass are normal. These arise from measurement uncertainties and small losses during separation. Errors of less than 1% are generally acceptable.

Significant figures: Your final percentages should have the same number of significant figures as your measurements. Don't claim more precision than your equipment can provide.

Complete separation: The accuracy of gravimetric analysis depends on completely separating the components. Any contamination of one component with another will affect your results.

Proper drying: Components must be completely dry before weighing, or your mass measurements will be too high.

Summary

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Key Points to Remember:

Gravimetric analysis determines the mass and percentage of each component in a mixture - it's quantitative analysis based on weighing.
The key formula is:

$\text{Percentage} = \frac{\text{mass of component}}{\text{total mass}} \times 100$

Separation techniques are essential - you need to isolate each component before you can weigh it accurately.
Check your work - the sum of component masses should equal (or be very close to) your starting mass.
Real-world applications - gravimetric analysis is used in mining, quality control, and product development to ensure mixtures have the correct composition.

Gravimetric Analysis (HSC SSCE Chemistry): Revision Notes