Molar Mass of Phosphoric Acid (H₃PO₄)
Learn how chemists calculate the molar mass of Phosphoric Acid (H₃PO₄), with a clear formula breakdown, worked steps, and study notes · IUPAC name: Phosphoric acid.
Quick answer
The molar mass of Phosphoric Acid (H₃PO₄) is
97.994g/mol
One mole of Phosphoric Acid therefore has a mass of 97.994 grams—the value you use for stoichiometry and laboratory preparation.
Reviewed for educational accuracy · Accuracy policy
- CAS Registry Number
- 7664-38-2
- PubChem CID
- 1004
- SMILES
- OP(=O)(O)O
Step-by-step calculation
Let's find the molar mass of Phosphoric Acid (H₃PO₄) together—step by step, as if you are seeing the formula for the first time.
Step 1 — Look at the chemical formula
The formula is H₃PO₄. Each letter stands for an element. The little number after a letter (the subscript) tells you how many atoms of that element are in one molecule or formula unit.
- 3 Hydrogen atoms (H)
- 1 Phosphorus atom (P)
- 4 Oxygen atoms (O)
Step 2 — Look up each atomic mass
Atomic mass comes from the periodic table. It is the average mass of one mole of atoms of that element, in grams per mole (g/mol). Think of it as the "price tag" for one mole of that element.
- Hydrogen (H) = 1.008 g/mol
- Phosphorus (P) = 30.974 g/mol
- Oxygen (O) = 15.999 g/mol
Step 3 — Multiply atoms × atomic mass
Why multiply? If one oxygen atom "costs" about 16 g/mol, then two oxygen atoms cost twice as much. Each element's contribution is: number of atoms × atomic mass.
- 3 × 1.008 = 3.024 g/mol (Hydrogen)
- 1 × 30.974 = 30.974 g/mol (Phosphorus)
- 4 × 15.999 = 63.996 g/mol (Oxygen)
Step 4 — Add the contributions
Why add? The molar mass of the whole compound is simply the total mass of every atom in the formula. Add each element's contribution:
3.024 + 30.974 + 63.996 = 97.994 g/mol
Step 5 — Final answer
Molar mass of Phosphoric Acid = 97.994 g/mol
That means one mole of Phosphoric Acid (H₃PO₄) has a mass of about 97.99 grams.
Quick summary
Read the formula → count atoms → look up atomic masses → multiply → add → report g/mol. For H₃PO₄, the total is 97.994 g/mol.
Common beginner mistakes
- Treating H₃PO₄ as strong acid — it is weak (partial dissociation).
- Using all three protons in neutralization at pH 7 — only ~1.5 equivalents typically.
- Confusing H₃PO₄ with H₃PO₃ (phosphorous acid, P in +3, diprotic).
Memory trick
Phosphate buffer: pH = pKa₂ + log([HPO₄²⁻]/[H₂PO₄⁻]).
Mini practice
Without looking above, list the atoms in H₃PO₄ and write one multiplication line for the heaviest element. Then check your work against Step 3.
Real-world example
If a recipe asks for 0.100 mol of Phosphoric Acid, mass needed = 0.100 × 97.994 = 9.799 g. That is how chemists turn a mole amount into a weighable sample.
Atomic contribution table
Each row shows how much mass one element contributes to the total for H₃PO₄.
| Element | Atoms | Atomic mass | Contribution | Mass % |
|---|---|---|---|---|
| H | 3 | 1.008 | 3.024 g/mol | 3.1% |
| P | 1 | 30.974 | 30.974 g/mol | 31.6% |
| O | 4 | 15.999 | 63.996 g/mol | 65.3% |
| Total molar mass | 97.994 g/mol | 100% | ||
Mass contribution chart
In water, many acids transfer H⁺ (a proton) to form hydronium. Molar mass still comes from the undissociated formula used to prepare the solution.
Download study sheets
Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Phosphoric Acid (H₃PO₄).
Practice this calculation
Without looking above, write the atom count for H₃PO₄, then compute the molar mass. Check your answer against 97.994 g/mol.
Next challenge: how many grams are in 0.250 mol of Phosphoric Acid? Multiply 0.250 × 97.994 to get 24.498 g.
Physical and chemical properties
Physical properties
| Appearance | Colorless viscous liquid (concentrated) or white solid (pure, below 42 °C) |
| Color | Colorless |
| Odor | Odorless |
| State (STP) | Liquid (85% solution common); pure melts at 42.4 °C |
| Density | 1.885 g/cm³ (85% solution, 25 °C) |
| Melting point | 42.4 °C (pure, forms hemihydrate) |
| Boiling point | Decomposes before boiling (213 °C loses water to form pyrophosphoric acid) |
| Solubility | Miscible with water in all proportions |
| Crystal structure | Monoclinic (solid H₃PO₄) |
Chemical properties
| Classification | Weak triprotic acid / oxoacid of phosphorus |
| Family | Oxoacids of phosphorus (orthophosphoric acid) |
| Acidity | Weak triprotic (pKa₁ = 2.15, pKa₂ = 7.20, pKa₃ = 12.35) |
| Polarity | Highly polar |
| Geometry | Tetrahedral about phosphorus |
| Bond angle | ~109.5° |
| Oxidation states | P: +5, O: −2, H: +1 |
Applications
Industrial uses
- Triple superphosphate and ammonium phosphate fertilizer production
- Rust removal and metal surface treatment
- Food acidulant in cola, jams, and processed cheese (E338)
- Detergent builder (sodium tripolyphosphate, being phased out)
Laboratory uses
- Buffer preparation (phosphate buffer, pH 6–8 range)
- Weak acid titration experiments (triprotic, multiple equivalence points)
- Electrolyte in some electrochemical cells
Phosphate backbone of DNA and RNA; ATP (adenosine triphosphate) energy currency; bone hydroxyapatite Ca₅(PO₄)₃OH.
Preparation and production
Wet process: sulfuric acid on phosphate rock (industrial). Thermal process: P₄ + 5 O₂ → P₄O₁₀; P₄O₁₀ + 6 H₂O → 4 H₃PO₄ (purer, food grade). Laboratory: PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl.
Global phosphoric acid production exceeds 45 million tonnes annually, overwhelmingly for fertilizer from wet process.
Important reactions of Phosphoric Acid
H₃PO₄ + 3 NaOH → Na₃PO₄ + 3 H₂O
- Reaction type
- Acid–base neutralization (complete)
- Conditions
- Excess strong base, high pH
- Explanation
- All three protons neutralized forming tribasic sodium phosphate; at lower pH, NaH₂PO₄ or Na₂HPO₄ forms.
- Products
- Trisodium phosphate and water
- Why it matters
- Phosphate salt production, water softening (historical detergents)
Related ideas: Triprotic acids · Multiple equivalence points · Salt types
Ca₅(PO₄)₃F + 5 H₂SO₄ + 10 H₂O → 3 H₃PO₄ + 5 CaSO₄·2H₂O + HF
- Reaction type
- Wet-process acid production
- Conditions
- Industrial, phosphate rock digestion
- Explanation
- Sulfuric acid liberates phosphoric acid from mineral apatite, producing gypsum byproduct.
- Products
- Phosphoric acid, gypsum, hydrogen fluoride
- Why it matters
- Industrial fertilizer acid production
Related ideas: Industrial chemistry · Mineral processing
H₃PO₄ ⇌ H₂PO₄⁻ + H⁺
- Reaction type
- First dissociation
- Conditions
- Aqueous, pH ~2–7
- Explanation
- First proton dissociates with pKa₁ = 2.15; dominates acid character in dilute solutions.
- Products
- Dihydrogen phosphate and hydrogen ion
- Why it matters
- Acidity in beverages, buffer systems
Related ideas: Weak acids · Equilibrium · pH
H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺
- Reaction type
- Second dissociation (buffer)
- Conditions
- pH 6–8, physiological
- Explanation
- Second dissociation pKa₂ = 7.20 — effective buffer pair H₂PO₄⁻/HPO₄²⁻ in blood and cells.
- Products
- Hydrogen phosphate and hydrogen ion
- Why it matters
- Biological buffering, laboratory phosphate buffers
Related ideas: Buffers · Henderson–Hasselbalch · Physiological chemistry
History and discovery
Scheele prepared phosphoric acid from bone ash (1771). Liebig recognized phosphate importance in plant nutrition (1840s), leading to fertilizer industry. Development of wet-process acid from phosphate rock (19th century) enabled modern agriculture.
Carl Wilhelm Scheele, 1771 — isolated phosphoric acid from heated bone ash (calcium phosphate) with nitric acid.
Interesting facts
- Cola pH ~2.5 comes partly from H₃PO₄ — molar mass 98 g/mol means trace mass but strong effect at low pH.
- Phosphate buffer (H₂PO₄⁻/HPO₄²⁻) is effective near pH 7.2, matching physiological pH.
- Your DNA contains roughly 3 billion phosphate groups linking nucleotides.
- Wet-process H₃PO₄ often contains fluoride and sulfate impurities; thermal process is purer.
Comparison with similar compounds
H₃PO₄ (98.00 g/mol) is weak triprotic and non-oxidizing; H₂SO₄ (98.08 g/mol, similar molar mass) is strong diprotic and dehydrating — very different chemistry despite similar mass.
Storage, handling, and safety
Store in corrosion-resistant containers (HDPE, rubber-lined steel). Concentrated acid is viscous and hygroscopic. Food grade separate from industrial grade.
Corrosive to eyes and skin. Use splash goggles and gloves. Food-grade handling follows food safety protocols; industrial grade requires chemical PPE.
Corrosive acid; eye damage risk. Food-grade at beverage concentrations is safe; concentrated form causes burns.
- Eye corrosion
- Skin irritation and burns (concentrated)
- Inhalation of mist irritates respiratory tract
Classification: GHS: Skin Corr. 1B (concentrated), Eye Dam. 1
Exam notes and student tips
Exam notes
- Molar mass H₃PO₄ = 3(1.008) + 30.97 + 4(16.00) = 98.00 g/mol.
- Triprotic: pKa₁ = 2.15, pKa₂ = 7.20, pKa₃ = 12.35.
- At pH 7.2, [H₂PO₄⁻] ≈ [HPO₄²⁻] (pH ≈ pKa₂).
- P oxidation state in H₃PO₄ is +5.
Student tips
- Phosphate buffer: pH = pKa₂ + log([HPO₄²⁻]/[H₂PO₄⁻]).
- For fertilizer stoichiometry, trace apatite + H₂SO₄ → H₃PO₄ pathway.
- Link PO₄³⁻ to bone mineral and DNA backbone for biochemistry crossover.
Common mistakes
- Treating H₃PO₄ as strong acid — it is weak (partial dissociation).
- Using all three protons in neutralization at pH 7 — only ~1.5 equivalents typically.
- Confusing H₃PO₄ with H₃PO₃ (phosphorous acid, P in +3, diprotic).
Misconceptions
- Cola dissolves teeth instantly — acidity contributes to erosion over time, not instant dissolution.
- All phosphate in fertilizer is immediately available — some precipitates as insoluble salts in soil.
- H₃PO₄ and phosphoric acid in food labels are the same compound at food-grade purity.
Practice questions
1. Calculate the molar mass of H₃PO₄.
Show answer
3(1.008) + 30.97 + 4(16.00) = 98.00 g/mol
2. What is the pH of 0.10 M H₃PO₄ (first dissociation only, pKa₁ = 2.15)?
Hint: Use weak acid approximation for first proton only.
Show answer
Ka₁ = 10^−2.15 ≈ 7.1 × 10⁻³; [H⁺] ≈ √(Ka₁ × C) ≈ 0.027 M; pH ≈ 1.57
3. What phosphate species dominates at pH 7.4 (blood)?
Show answer
Mixture of H₂PO₄⁻ and HPO₄²⁻ near 1:1 ratio since pH ≈ pKa₂ (7.20).
4. What is phosphorus oxidation state in H₃PO₄?
Show answer
+5
Frequently asked questions about Phosphoric Acid
98.00 g/mol.
Chemistry of Phosphoric Acid
The sections above give the number you need for calculations. Here we look more closely at how Phosphoric Acid (H₃PO₄) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.
Phosphoric acid (H₃PO₄) is a triprotic acid with molar mass 98.00 g/mol (H 3.024 + P 30.97 + O 64.00), the most important oxoacid of phosphorus. Unlike sulfuric and nitric acids, H₃PO₄ is a weak acid with three dissociation steps: pKa₁ = 2.15, pKa₂ = 7.20, pKa₃ = 12.35. At physiological pH, phosphate exists mainly as HPO₄²⁻ and H₂PO₄⁻ — the buffer pair in blood and intracellular fluids.
Food-grade H₃PO₄ gives cola drinks their tangy flavor and acidity (pH ~2.5 in soft drinks). Industrially, H₃PO₄ is produced from phosphate rock (fluoroapatite, Ca₅(PO₄)₃F) treated with sulfuric acid: Ca₅(PO₄)₃F + 5 H₂SO₄ + 10 H₂O → 3 H₃PO₄ + 5 CaSO₄·2H₂O + HF. Global fertilizer industry depends on this wet-process phosphoric acid.
H₃PO₄ contains phosphorus in +5 oxidation state with tetrahedral PO₄ geometry. Three hydroxyl groups bear acidic protons; the P=O bond is the fourth oxygen. Successive deprotonations yield H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻. The formula indicates three replaceable hydrogens, though only the first two are relevant near neutral pH.
H₃PO₄ is weakly acidic and non-oxidizing — it does not char organic matter like H₂SO₄. It forms phosphate salts with metals (Na₃PO₄, Ca₃(PO₄)₂), complexes with metal ions, and condenses to pyrophosphoric (H₄P₂O₇) and metaphosphoric acids when heated. With alcohols it forms phosphate esters (ATP biologically).
The tangy acidulant behind cola and soft drinks
Food-grade phosphoric acid gives cola beverages their characteristic sharp, tangy edge and helps balance the sweetness of high fructose corn syrup, typically bringing soft drink pH down to around 2.5. Because H₃PO₄ is only weakly dissociated, a relatively small amount produces this acidity without dominating the overall flavor profile the way a stronger, fully dissociated acid would.
Rust removal and metal surface treatment chemistry
Phosphoric acid dissolves iron oxide (rust) by converting it to soluble iron phosphate species, while simultaneously depositing a thin, adherent iron phosphate layer on the underlying clean metal surface that resists future corrosion — the basis of both consumer rust-removal products and industrial phosphating/parkerizing metal pretreatment processes used before painting or coating steel parts.
Weak triprotic acid: a distinctive titration profile
Unlike strong monoprotic or diprotic acids, phosphoric acid's three widely separated pKa values (2.15, 7.20, 12.35) produce a titration curve with three distinct buffering regions and equivalence points, making H₃PO₄ a favorite teaching example for illustrating how polyprotic weak acids behave very differently from strong acids like HCl or H₂SO₄ during stepwise neutralization.
Triple superphosphate and the fertilizer connection
Concentrated phosphoric acid reacts with phosphate rock to produce triple superphosphate, a highly concentrated, water-soluble phosphate fertilizer that supplies far more available phosphorus per unit mass than the ordinary superphosphate made with sulfuric acid alone — a key link between phosphoric acid's industrial-scale production and global agricultural productivity.
Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the acid library.
References and further reading
- USGS: Phosphate rock and phosphoric acid statistics
- PubChem CID 1004: Phosphoric acid data
- NIST Chemistry WebBook: Thermodynamic properties