Molar Mass of Aluminum Sulfate (Al₂(SO₄)₃)

Each row shows how much mass one element contributes to the total for Al₂(SO₄)₃.

Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Aluminum Sulfate (Al₂(SO₄)₃).

Without looking above, write the atom count for Al₂(SO₄)₃, then compute the molar mass. Check your answer against 342.132 g/mol.

Next challenge: how many grams are in 0.250 mol of Aluminum Sulfate? Multiply 0.250 × 342.132 to get 85.533 g.

Widely used in drinking water and wastewater treatment to remove turbidity, pathogens, and phosphorus; residual aluminum in treated water and sludge disposal are managed under water quality regulations.

Not used biologically as a nutrient; aluminum hydroxide gels derived from it have limited medicinal use as antacids and vaccine adjuvants in some formulations.

Preparation and production

Industrially produced by reacting bauxite or purified aluminum hydroxide with sulfuric acid: 2 Al(OH)₃ + 3 H₂SO₄ → Al₂(SO₄)₃ + 6 H₂O, followed by crystallization to the desired hydrate form. Laboratory-grade material can be prepared similarly from aluminum oxide or hydroxide with sulfuric acid.

Global production is substantial given its role as the dominant low-cost water treatment coagulant; major producers supply both liquid alum solutions and solid hydrate forms to municipal utilities and industrial customers worldwide.

Aluminum sulfate and related alum compounds have been used since antiquity as mordants in dyeing and in early water clarification practices. Its systematic use as a municipal water treatment coagulant expanded dramatically in the late 19th and early 20th centuries as public health authorities recognized the link between water clarity, pathogen removal, and disease prevention, cementing its role as a cornerstone of modern water treatment engineering.

Alum compounds used since antiquity in dyeing and water clarification; aluminum sulfate's chemical identity clarified alongside broader 18th–19th century advances in inorganic chemistry.

Interesting facts

• Aluminum sulfate's coagulation chemistry has been used to purify drinking water since at least the 19th century, long before the detailed aluminum hydrolysis chemistry was understood.
• A single water treatment plant can use many tonnes of aluminum sulfate per day to clarify municipal drinking water supplies.
• The mild acidity of dissolved aluminum sulfate is strong enough that gardeners use it to acidify soil for acid-loving plants like hydrangeas, shifting their flower color toward blue.
• Papermaker's alum was historically so closely tied to the paper industry that the terms became almost synonymous in some trade contexts.

Comparison with similar compounds

Aluminum sulfate (Al₂(SO₄)₃, 342.15 g/mol) is the primary coagulant salt used in water treatment, distinct from potassium alum (KAl(SO₄)₂·12H₂O, 474.39 g/mol), a related double sulfate more commonly associated with historical dyeing, pickling, and styptic pencil applications.

Store the solid hydrate in a dry, corrosion-resistant container, as it can cake or partially dissolve in humid conditions. Liquid alum solutions should be stored in acid-resistant tanks, as the mildly acidic solution can corrode standard metals over time.

Mild to moderate irritant; concentrated solutions are acidic and can irritate skin, eyes, and respiratory tract. Use gloves and eye protection when handling bulk solid or concentrated solution.

Generally low acute toxicity; concentrated solutions are mildly acidic and can cause skin and eye irritation. Widely and safely used in drinking water treatment at regulated dosing levels.

• Eye and skin irritation from concentrated solution or dust
• Respiratory irritation from inhaled powder
• Mildly corrosive to some metals in solution form due to acidity

Classification: GHS: Eye Irrit. 2, Skin Irrit. 2 (concentrated material)

1. 1. Calculate the molar mass of Al₂(SO₄)₃.

   Show answer

   2(26.98) + 3[32.06 + 4(16.00)] = 53.96 + 288.18 = 342.14 g/mol

2. 2. How many moles of Al³⁺ are released when 34.21 g of Al₂(SO₄)₃ fully dissolves?

   Show answer

   34.21 g ÷ 342.14 g/mol ≈ 0.100 mol Al₂(SO₄)₃ → 0.100 × 2 = 0.200 mol Al³⁺

3. 3. How many moles of NaOH are needed to fully precipitate the aluminum in 0.0500 mol Al₂(SO₄)₃ as Al(OH)₃?

   Show answer

   0.0500 mol Al₂(SO₄)₃ contains 0.100 mol Al³⁺; each requires 3 mol OH⁻, so 0.100 × 3 = 0.300 mol NaOH

4. 4. Why does adding aluminum sulfate to water lower its pH?

   Show answer

   The Al³⁺ ion hydrolyzes water molecules, releasing H⁺ ions as it forms aluminum hydroxide species, making the solution more acidic.

• Sulfuric AcidH₂SO₄

The sections above give the number you need for calculations. Here we look more closely at how Aluminum Sulfate (Al₂(SO₄)₃) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.

Aluminum sulfate (Al₂(SO₄)₃) is an ionic compound with molar mass 342.15 g/mol (Al 2 × 26.98 + S 3 × 32.06 + O 12 × 16.00), formed from two Al³⁺ ions balancing three SO₄²⁻ ions. Commercial "alum" used in water treatment is most commonly the hydrate Al₂(SO₄)₃·14H₂O or Al₂(SO₄)₃·18H₂O, a white crystalline solid that dissolves readily in water to give mildly acidic solutions due to hydrolysis of the small, highly charged Al³⁺ ion.

By far the largest use of aluminum sulfate is as a coagulant in municipal and industrial water treatment: when added to raw water, Al³⁺ hydrolyzes to form positively charged aluminum hydroxide polymers that neutralize the negative surface charge on suspended clay, silt, and organic particles, allowing them to aggregate into larger flocs (flocculation) that settle out or are filtered away — a process that has purified drinking water and treated wastewater at massive scale since the 19th century and remains one of the most widely used coagulants worldwide because of its low cost and reliable performance across a range of water chemistries.

In the paper industry, aluminum sulfate serves a related but distinct purpose called sizing: added during paper manufacture, it helps precipitate and fix rosin-based sizing agents onto cellulose fibers, controlling how readily the finished paper absorbs ink and moisture. This once-dominant "acid papermaking" process, still used for many grades of paper, exploits the same aluminum hydrolysis chemistry that drives water treatment coagulation, illustrating how a single compound's ion chemistry underlies two seemingly unrelated industrial processes.

Al₂(SO₄)₃ balances two Al³⁺ ions (total charge +6) against three SO₄²⁻ ions (total charge −6). Each sulfate ion is a tetrahedral anion with sulfur in the +6 oxidation state. The formula's 2:3 ratio directly reflects the need to equalize the +3 charge of aluminum with the −2 charge of sulfate through their lowest common multiple.

Al₂(SO₄)₃ dissolves in water to give an acidic solution (pH typically 3–4 for concentrated solutions) because the small, highly charged Al³⁺ ion strongly polarizes coordinated water molecules, promoting their deprotonation (hydrolysis) to release H⁺. This hydrolysis generates positively charged aluminum hydroxide species that are the active coagulating agents in water treatment. Al₂(SO₄)₃ reacts with bases to precipitate aluminum hydroxide, which is itself amphoteric and redissolves in excess strong base to form aluminate ion.

Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the salt library.

• PubChem CID 24850: Aluminum sulfate compound data
• American Water Works Association: Coagulation and flocculation treatment guidance
• NIST Chemistry WebBook: Thermodynamic properties