Quick Notes

Quick Notes

Chemistry Chapter 6: Acid, Base and Salt
6.6 Standard Solution
6.6 Standard Solution - Chemistry Form 4
CHEMISTRY • Form 4 • Chapter 6: Acid, Base and Salt

6.6Standard Solution

A standard solution is a vital concept in quantitative chemistry because chemical reactions often occur between reactants in aqueous solutions. Just like a commercial syrup dispenser that delivers precise amounts of sugar syrup of known sweetness, chemists require solutions with accurately known concentrations to perform precise experiments and chemical analyses. Standard solutions can be prepared either by dissolving a known mass of a solid solute or by diluting an existing concentrated stock solution.

Learning Objectives

  • State the meaning of a standard solution.
  • Describe the preparation of a standard solution from a solid substance through an activity.
  • Describe the preparation of a standard solution through the dilution of an aqueous solution.
  • Explain why certain substances like sodium hydroxide are unsuitable for preparing standard solutions directly.
  • Solve numerical problems involving the preparation of standard solutions and the dilution formula \(M_1V_1 = M_2V_2\).

Summary of Standard Solution Preparation: Dissolution vs Dilution

Compare the step-by-step procedures for preparing a standard solution from a solid solute and by dilution of a stock solution.

From Solid

Weigh the calculated mass → dissolve in a beaker → transfer into a volumetric flask → rinse beaker and funnel → top up with a dropper → invert to mix.

By Dilution

Calculate \(V_1\) → pipette the stock solution → transfer into a volumetric flask → top up with a dropper → invert to mix.

Key idea: In both methods, the final volume is measured accurately using a volumetric flask. The final adjustment to the calibration mark must always be done dropwise using a dropper at eye level so that the bottom of the meniscus aligns exactly with the mark.

Brief Explanation

Understand the key ideas behind standard solutions and their preparation.

A standard solution is a solution with a known concentration. To prepare a standard solution, the mass of solute must be measured accurately and the solution made up accurately to the required final volume in a volumetric flask.

Preparation from a Solid

Example: preparing \(250\text{ cm}^3\) of \(1.0\text{ mol dm}^{-3}\) sodium carbonate solution, \(\text{Na}_2\text{CO}_3\).

Number of moles: \(n = \dfrac{MV}{1000}\)

Mass: \(\text{Mass} = n \times \text{molar mass}\)

Key steps: weigh → dissolve → transfer → rinse → top up with dropper → invert.

Preparation by Dilution

Dilution involves adding distilled water to a concentrated stock solution to produce a more dilute solution.

Adding water changes the concentration and total volume, but the number of moles of solute remains constant:

\(M_1V_1 = M_2V_2\)

Quantity Meaning
\(M_1\) Molarity of stock solution before dilution
\(V_1\) Volume of stock solution needed
\(M_2\) Molarity of prepared solution after dilution
\(V_2\) Volume of prepared solution after dilution
Suitability of substances: Solid hygroscopic sodium hydroxide, \(\text{NaOH}\), is not suitable to prepare a standard solution directly because it absorbs water and carbon dioxide gas from the air to form sodium carbonate: \[2\text{NaOH(s)} + \text{CO}_2\text{(g)} \rightarrow \text{Na}_2\text{CO}_3\text{(s)} + \text{H}_2\text{O(l)}\] This makes it difficult to determine the exact mass of pure \(\text{NaOH}\). Solid oxalic acid, \(\text{H}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}\), can be used to prepare a standard solution in the laboratory.

Try These Questions First

Answer mentally, then check the explanation.

1

What is the definition of a standard solution?

Answer: A solution with a known concentration.
2

Why is solid sodium hydroxide, \(\text{NaOH}\), unsuitable for preparing a standard solution?

Answer: Sodium hydroxide is hygroscopic (absorbs moisture from air) and absorbs carbon dioxide from the air to form sodium carbonate, making it difficult to determine its exact mass.
3

Calculate the volume of \(2.0\text{ mol dm}^{-3}\) hydrochloric acid, \(\text{HCl}\), needed to prepare \(250\text{ cm}^3\) of \(0.2\text{ mol dm}^{-3}\) hydrochloric acid.

Answer: Using \(M_1V_1 = M_2V_2\): \((2.0)(V_1) = (0.2)(250)\), so \(V_1 = \dfrac{0.2 \times 250}{2.0} = 25\text{ cm}^3\).

Common Mistakes

Avoid these errors when preparing standard solutions.

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Rinsing the pipette with distilled water immediately before drawing the stock solution. This introduces extra water and dilutes the stock solution. The pipette should be rinsed with the stock solution.
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Forgetting to rinse the beaker and filter funnel with distilled water when transferring dissolved solid into the volumetric flask.
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Adding water past the calibration mark of the volumetric flask directly from a wash bottle instead of using a dropper for the final drops.
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Blowing out the residual drop of liquid left in the tip of the pipette.
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Using a beaker instead of a volumetric flask to measure the final solution volume.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Adding water during dilution decreases the number of moles of solute.

Correct Fact

Dilution decreases the concentration of the solution, but the total number of moles of solute remains constant (\(n_1 = n_2\)).

Misconception

In \(M_1V_1 = M_2V_2\), \(V_2\) represents the volume of water added.

Correct Fact

\(V_2\) is the final total volume of the diluted solution (\(V_2 = V_1 + V_{\text{water added}}\)), not just the volume of water added.

Misconception

The calibration mark can be aligned with the top edges of the water meniscus.

Correct Fact

The bottom of the curved meniscus must be aligned with the calibration mark at eye level.

Summary

Key facts to remember.

  • A standard solution is a solution whose concentration is accurately known.
  • Preparation from a solid involves calculating mass (\(n = \dfrac{MV}{1000}\)), weighing, dissolving in a beaker, transferring into a volumetric flask, rinsing apparatus, making up to the calibration mark with a dropper, and inverting.
  • Solid \(\text{NaOH}\) cannot be used directly because it is hygroscopic and reacts with atmospheric \(\text{CO}_2\).
  • Dilution relies on the conservation of solute moles: \(M_1V_1 = M_2V_2\).
  • A pipette is used to deliver a precise volume of stock solution, and a volumetric flask is used to measure the final volume accurately.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Why must the beaker and filter funnel be rinsed with distilled water during the preparation of a standard solution from a solid?

2

What is the volume of distilled water that must be added to \(75\text{ cm}^3\) of \(2.0\text{ mol dm}^{-3}\) nitric acid, \(\text{HNO}_3\), to produce a \(1.5\text{ mol dm}^{-3}\) nitric acid solution?

B. Fill in the Blanks

3

During the dilution of a stock solution, adding distilled water changes the concentration but does not alter the number of ______ of solute.

4

Sodium hydroxide is not suitable for preparing a standard solution because it absorbs water and is described as being ______.

C. Match the Apparatus with Its Function

Drag each apparatus on desktop. On mobile, tap an apparatus and then tap the matching box.

Apparatus

Volumetric flask Pipette Dropper Stopper

Function

1Holds and accurately measures a fixed total volume of standard solution at the calibration mark
2Accurately draws and delivers a specific fixed volume of stock solution
3Adds distilled water slowly drop by drop to align the meniscus with the calibration mark
4Seals the volumetric flask so it can be inverted to produce a homogeneous solution
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Standard solution Stock solution Dilution Volumetric flask Calibration mark Meniscus Hygroscopic Homogeneous