What Is the Molar Mass of Phosphoric Acid? | Done Right

Phosphoric acid (H3PO4) has a molar mass of 98.00 g/mol.

If you’re trying to balance an equation, mix a solution, or check a lab worksheet, the molar mass is the number that keeps everything honest. For phosphoric acid, the target is simple: 98.00 grams per mole. The part that trips people up is the path to that number—where each atom count comes from, which atomic masses to use, and how to sanity-check the result.

This walkthrough gives you the clean math, plus a few fast checks that catch the common slip-ups. You’ll also see how to handle rounding, how to deal with hydrates and solution labels, and how to reuse the same method for any formula you meet next.

What “Molar Mass” Means In One Line

Molar mass is the mass of 1 mole of a substance, measured in g/mol. A mole is a count, like a dozen—just with a much larger number of particles. So molar mass is a “grams per mole” conversion factor between what you can weigh and what chemistry counts.

When you see H3PO4, you’re looking at a recipe: 3 hydrogen atoms, 1 phosphorus atom, and 4 oxygen atoms in each formula unit of phosphoric acid.

Molar Mass Of Phosphoric Acid With Step-By-Step Math

Start with the chemical formula:

• H3PO4
• Atoms per formula unit: H = 3, P = 1, O = 4

Next, pull the standard atomic masses (the values on a periodic table). You’ll see small variation across sources because atomic masses reflect natural isotope mixes. For classroom and lab work, using a consistent, reputable table is what matters.

Here are commonly used standard atomic masses to two decimal places:

• Hydrogen (H) = 1.01 g/mol
• Phosphorus (P) = 30.97 g/mol
• Oxygen (O) = 16.00 g/mol

Now multiply each atomic mass by the atom count in the formula, then add the parts:

1. Hydrogen: 3 × 1.01 = 3.03 g/mol
2. Phosphorus: 1 × 30.97 = 30.97 g/mol
3. Oxygen: 4 × 16.00 = 64.00 g/mol
4. Total: 3.03 + 30.97 + 64.00 = 98.00 g/mol

That’s the full computation. If your periodic table uses slightly different decimals, your final value may land near 98.0 g/mol with a tiny shift in the last digit. The method stays the same.

Rounding That Matches Real Class And Lab Expectations

Most classes accept 98.00 g/mol, 98.0 g/mol, or 98 g/mol depending on the rounding rules your instructor uses. In lab settings, match your sig figs to the data you were given. If your assignment provides atomic masses with two decimals, report the final molar mass with two decimals.

If your periodic table lists hydrogen as 1.008, oxygen as 15.999, and phosphorus as 30.974, you can run the same math and keep more digits through the final addition. The end value still sits right at 97.99–98.00 g/mol when rounded to two decimals.

Quick Checks That Catch Mistakes Early

You can spot most errors before you ever reach the last line.

• Order-of-size check: Four oxygens already give you about 64 g/mol. Add phosphorus at about 31 g/mol and you’re near 95 g/mol before counting hydrogen. So the final answer should land near 98, not near 49 or 196.
• Atom-count check: The “3” applies only to H. The “4” applies only to O. P has no subscript, so it’s 1.
• Mass-share check: Oxygen should be the largest slice of the total because you have four of them.

Those checks help even when you’re tired, rushing, or copying a formula from a worksheet.

Atomic Mass Choices And Where They Come From

Atomic masses on the periodic table are weighted averages. That’s why they often show decimals. If you want a reputable reference for these values, you can pull element masses from an official data source like NIST relative atomic masses. If you want the formal standard atomic weights and how they’re maintained, you can also check IUPAC standard atomic weights.

For most homework and many routine lab calculations, the periodic table values printed in your lab manual are the ones to use, since graders and lab partners are using the same sheet.

Composition Breakdown For H3PO4

The table below shows each element’s contribution to the total molar mass. This is the cleanest way to audit your work, since you can see the “parts list” and the total at a glance.

Element Part	How It’s Calculated	Mass Contribution (g/mol)
Hydrogen (H)	3 × 1.01	3.03
Phosphorus (P)	1 × 30.97	30.97
Oxygen (O)	4 × 16.00	64.00
Total	3.03 + 30.97 + 64.00	98.00
Oxygen Share	64.00 ÷ 98.00	0.653
Phosphorus Share	30.97 ÷ 98.00	0.316
Hydrogen Share	3.03 ÷ 98.00	0.031

The “share” lines are optional, but they’re handy in word problems that ask for percent composition or when you want a quick gut-check: oxygen should be near two-thirds of the mass, phosphorus near one-third, and hydrogen only a small slice.

Using The Molar Mass In Real Calculations

Once you trust the 98.00 g/mol value, you can swap between grams and moles without stress.

Grams To Moles

To convert grams of phosphoric acid to moles, divide by 98.00 g/mol.

• If you have 49.0 g of H3PO4: moles = 49.0 ÷ 98.00 = 0.500 mol

Moles To Grams

To convert moles to grams, multiply by 98.00 g/mol.

• If you have 0.250 mol of H3PO4: grams = 0.250 × 98.00 = 24.5 g

These conversions show up everywhere: stoichiometry, titrations, solution prep, and percent by mass problems.

Solution Labels And What “Percent” Often Means

Phosphoric acid is often sold as an aqueous solution, not as pure H3PO4 crystals. Bottles may list a percent like “85%”. That number is commonly mass percent (w/w), meaning 85 g of H3PO4 per 100 g of solution. It is not molarity. It is not “85% pure by volume.”

If you’re given mass percent and density, you can compute molarity, but that’s a separate step from molar mass. Molar mass stays 98.00 g/mol for H3PO4, no matter the concentration.

Hydrates, Additives, And Similar Formulas That Look Close

Some worksheets mix in related formulas that can fool your eyes. Two common slips:

• Swapping the formula: H3PO3 (phosphorous acid) is not the same as H3PO4. One oxygen difference changes molar mass by about 16 g/mol.
• Ignoring water of hydration: If a compound is written with “·nH2O”, that water adds real mass. You must include it in the molar mass.

When you see a dot, treat it like a plus sign for mass. Count every atom on both sides of the dot.

Common Error Patterns And How To Fix Them

Most wrong answers come from a short list of habits. The good news: each one has a simple fix.

• Forgetting the “1” subscript: P has no number, so it’s 1. Write it as P1 while you work, then drop the 1 at the end.
• Multiplying the wrong element: The “3” is only for H, and the “4” is only for O. Recopy the formula with spacing: H H H P O O O O.
• Rounding too early: Keep extra digits until the final sum if your atomic masses have extra digits.
• Mixing units: Atomic masses are in g/mol for elements, and the final molar mass is in g/mol for the compound.

If you correct those four, your molar-mass work gets steady fast.

Fast Reference Table For Related Substances

If you’re working a set of problems, phosphoric acid often sits next to similar compounds. This table is a compact reference for spotting differences in oxygen count or ion form. Values below use common periodic-table atomic masses to two decimals.

Formula	Name	Molar Mass (g/mol)
H3PO4	Phosphoric acid	98.00
H3PO3	Phosphorous acid	82.00
H3PO2	Hypophosphorous acid	66.00
PO4^3−	Phosphate ion	94.97
H2PO4^−	Dihydrogen phosphate ion	97.00
HPO4^2−	Hydrogen phosphate ion	96.00

Notice how close some of these values are. A one-hydrogen difference shifts the mass by about 1.01 g/mol. A one-oxygen difference shifts the mass by about 16.00 g/mol. That pattern is a clean way to check your math across a problem set.

Reusable Template You Can Copy For Any Formula

When you meet a new chemical formula, run the same loop:

1. Write the formula and list each element with its subscript count.
2. Pull atomic masses from one consistent table.
3. Multiply each atomic mass by its count.
4. Add the contributions and keep units as g/mol.
5. Run a size check: does the total match what the big elements suggest?

That’s it. This method works for salts, acids, bases, and organic formulas. It also scales cleanly to parentheses. If you see something like Ca(OH)2, the “2” multiplies everything inside the parentheses.

Final Checklist Before You Turn In The Answer

• You counted atoms from the formula: H = 3, P = 1, O = 4.
• You multiplied each atomic mass by the right count.
• Your oxygen mass landed near 64 g/mol.
• Your total landed near 98 g/mol.
• Your reported value matches the rounding style used in your class or lab sheet.

If all five are true, your molar mass for phosphoric acid is ready to use: 98.00 g/mol.