Boiling Point Elevation Calculator
Calculate boiling point elevation, molality, the van't Hoff factor, the ebullioscopic constant, or an unknown solute's molar mass using ΔTb = i × Kb × m, with full step-by-step working.
Choose what to solve for, pick a solvent, then enter the known values.
Boiling point elevation (ΔTb)
Formula used: ΔTb = i × Kb × m
0.256 °C
Boiling point rise
100.256 °C
New boiling point
0.5 mol/kg
Molality
0.512 °C·kg/mol
Ebullioscopic constant
Interactive Boiling Flask & Thermometer
A live view of dissolved solute particles and the resulting rise in boiling point.
Step-by-Step Boiling Point Elevation Calculation
Here's exactly how this answer was calculated, one step at a time.
Given: m = 0.5 mol/kg, i = 1, solvent = Water (H₂O)
Step 1: Start with the colligative property equation
Boiling point elevation depends only on how many dissolved particles are present per kilogram of solvent, not on what the solute is.
ΔTb = i × Kb × mStep 2: Identify the known values
i = 1, Kb = 0.512 °C·kg/mol, m = 0.5 mol/kgStep 3: Substitute into ΔTb = i × Kb × m
ΔTb = 1 × 0.512 × 0.5Step 4: Report the result
ΔTb = 0.256 °C, so the new boiling point is 100.256 °C
Calculated result:
0.256 °C
Boiling Point Elevation Calculator: Find ΔTb Online
This free boiling point elevation calculator works out how much a dissolved solute raises the boiling point of a liquid, using the equation ΔTb = i × Kb × m. It also works backwards: give it a measured boiling point rise and it can find the molality, the van't Hoff factor, the ebullioscopic constant, or even the molar mass of an unknown solute. It is built for chemistry students, lab technicians, and anyone checking a colligative properties question or a homework answer.
Just pick what you want to solve for, choose a solvent from the list or enter your own Kb and boiling point, type in the values you already know, and the calculator shows the answer along with the full step-by-step working. No sign-up, no downloads, and no ads blocking the tool itself.
What Is Boiling Point Elevation?
Boiling point elevation is the rise in a liquid's boiling point that happens when you dissolve something in it. Pure water boils at 100 °C at normal atmospheric pressure, but salt water boils a little higher than that, because the dissolved salt gets in the way of water molecules escaping into vapor. This is a colligative property, which means it depends on how many particles are dissolved, not on what those particles actually are.
This is different from a chemical reaction. The solute does not react with the solvent; it simply sits between the solvent molecules and lowers the vapor pressure of the liquid at any given temperature. Because vapor pressure has to reach atmospheric pressure before boiling starts, the liquid needs a slightly higher temperature to boil once something is dissolved in it.
Boiling Point Elevation Formula: ΔTb = i × Kb × m
The core formula is ΔTb = i × Kb × m. ΔTb is the rise in boiling point in °C, i is the van't Hoff factor, Kb is the ebullioscopic constant of the solvent in °C·kg/mol, and m is the molality of the solution in mol/kg. Molality is used here instead of molarity because molality is based on the mass of the solvent, which does not shift as the solution is heated.
For example, dissolving 1 mole of sugar in 1 kg of water gives a 1 molal solution. Sugar does not split into ions in water, so i = 1. Using water's Kb of 0.512 °C·kg/mol, the boiling point rises by 0.512 °C, from 100 °C to about 100.512 °C. Small numbers like this are normal — boiling point elevation is a gentle effect for everyday concentrations.
What Is the Ebullioscopic Constant (Kb)?
The ebullioscopic constant, Kb, is a fixed property of each solvent. It tells you how much the boiling point rises for every 1 molal increase in concentration, assuming the solute does not split into ions. Water has a small Kb of 0.512 °C·kg/mol, while solvents like chloroform (3.63) or carbon tetrachloride (5.03) have much larger constants, so the same molality produces a bigger boiling point shift in those solvents.
Kb values are usually looked up in a reference table rather than calculated from scratch, since they depend on the solvent's molar mass, its normal boiling point, and its enthalpy of vaporization. This calculator includes preset Kb values for common lab solvents, and it also lets you calculate an unknown solvent's Kb from experimental data if you already have a measured ΔTb, molality, and van't Hoff factor.
What Is the Van't Hoff Factor?
The van't Hoff factor, i, tells you how many particles one unit of solute produces once it dissolves. A molecular solute that stays whole, such as sugar or urea, has i = 1. An ionic compound that fully separates into ions has a higher i: sodium chloride splits into Na+ and Cl-, giving i = 2, while calcium chloride splits into three ions total (one Ca2+ and two Cl-), giving i = 3.
This matters a lot for boiling point elevation, because a salt solution at the same molality as a sugar solution will raise the boiling point roughly two or three times as much, simply because it produces more particles per formula unit dissolved. Real solutions often show a slightly lower effective i than the ideal value because of ion pairing, especially at higher concentrations, but for classroom problems the ideal value is what's normally expected.
How to Calculate Boiling Point Elevation Step by Step
Start by identifying the molality of the solution — moles of solute divided by kilograms of solvent. Next, identify the van't Hoff factor based on whether the solute is molecular or ionic. Then look up or measure the solvent's ebullioscopic constant. Multiply all three together: ΔTb = i × Kb × m. Add that result to the solvent's normal boiling point to get the new boiling point of the solution.
As a worked example: dissolve 0.30 mol of NaCl in 0.50 kg of water. Molality is 0.30 ÷ 0.50 = 0.60 mol/kg. Since NaCl splits into 2 ions, i = 2. With Kb = 0.512 °C·kg/mol for water, ΔTb = 2 × 0.512 × 0.60 = 0.614 °C. The solution should start boiling at about 100.614 °C instead of 100 °C. Switch this calculator to the elevation mode and enter these same numbers to see the identical result with full working shown.
Finding an Unknown Molar Mass From Boiling Point Elevation
One of the most useful lab applications of this formula is finding the molar mass of an unknown compound. This technique is called ebullioscopy. Dissolve a known mass of the unknown solute in a known mass of solvent, measure the boiling point rise, and then rearrange the formula to solve for molar mass: M = (i × Kb × mass of solute) / (ΔTb × kg of solvent).
For instance, dissolving 5.00 g of an unknown non-ionic compound in 100 g (0.100 kg) of water produces a ΔTb of 0.256 °C. With i = 1 and Kb = 0.512, the molar mass works out to (1 × 0.512 × 5.00) / (0.256 × 0.100) = 100 g/mol. This calculator has a dedicated molar mass mode built for exactly this kind of lab problem — just enter the solute mass, solvent mass, and measured ΔTb, and it does the rearranged algebra for you with every step shown.
Boiling Point Elevation vs Freezing Point Depression
Boiling point elevation and freezing point depression are two sides of the same colligative-properties coin. Dissolving a solute raises the boiling point (ΔTb = iKbm) and lowers the freezing point (ΔTf = iKfm) at the same time, for the same underlying reason: the solute lowers the vapor pressure and disrupts how easily the solvent can form an organized solid or escape as a gas.
The two effects usually have very different sizes for the same solvent, because Kf and Kb are not equal. Water's Kf is 1.86 °C·kg/mol, over three times larger than its Kb of 0.512 °C·kg/mol. That's why salting an icy road has a much bigger effect on the freezing point than the same amount of salt would have on the boiling point of water on a stove.
Common Solvents and Their Boiling Point Constants
This calculator includes preset Kb values and normal boiling points for water (Kb 0.512, bp 100.0 °C), benzene (Kb 2.53, bp 80.1 °C), chloroform (Kb 3.63, bp 61.2 °C), ethanol (Kb 1.22, bp 78.4 °C), diethyl ether (Kb 2.02, bp 34.6 °C), glacial acetic acid (Kb 3.07, bp 118.1 °C), carbon tetrachloride (Kb 5.03, bp 76.7 °C), and cyclohexane (Kb 2.79, bp 80.7 °C).
If your solvent is not on the list, or your instructor has given you a different experimental value, switch to the custom solvent option and enter your own Kb and pure boiling point. The rest of the calculation works exactly the same way regardless of which solvent you choose.
Real-World Examples of Boiling Point Elevation
Salting pasta water is the classic kitchen example: the small amount of salt dissolved in a pot of water raises its boiling point by a fraction of a degree, far too small to noticeably speed up cooking, but it does happen. Engine coolant mixtures use the same principle deliberately — mixing ethylene glycol into water raises the boiling point of the coolant, helping it stay liquid at higher engine temperatures without boiling over.
It's worth noting this is different from why water boils at a lower temperature at high altitude. That change is caused by lower atmospheric pressure, not by anything dissolved in the water, so it isn't a colligative property in the sense covered by this calculator. Boiling point elevation specifically refers to the effect of a dissolved solute at constant pressure.
Common Mistakes When Calculating Boiling Point Elevation
The most common error is forgetting the van't Hoff factor for ionic solutes. Leaving i at 1 for a salt like NaCl or MgCl2 will give an answer that is far too small, since these compounds produce two or three particles per formula unit when they dissolve. Always check whether the solute is molecular or ionic before choosing i.
Another frequent mistake is mixing up molarity and molality, or using the mass of the whole solution instead of just the solvent. The Kb equation specifically needs molality, based on solvent mass alone. A third common slip is unit mismatch — mixing grams and kilograms, or forgetting to convert milligrams — which is why this calculator includes unit selectors on every mass field to help avoid that kind of error.
A Second Worked Example: Solving for Molality From ΔTb
Sometimes a question gives you the boiling point rise and asks you to work backwards to find the concentration. Suppose a solution of an unknown molecular compound in benzene boils at 81.35 °C instead of benzene's normal 80.1 °C, so ΔTb = 1.25 °C. Benzene's Kb is 2.53 °C·kg/mol, and since the solute is molecular, i = 1.
Rearranging the formula gives m = ΔTb / (i × Kb) = 1.25 / (1 × 2.53) = 0.494 mol/kg. Switch this calculator to the molality mode, enter ΔTb = 1.25, choose benzene as the solvent, and set i = 1, and it will return the same 0.494 mol/kg result along with the full rearranged working, so you can see exactly how each number was used.
Why Use an Online Boiling Point Elevation Calculator
Doing this math by hand is not hard once you know the formula, but it is easy to slip up on a unit conversion, forget the van't Hoff factor, or make a small arithmetic error, especially during an exam or a busy lab session. A calculator built specifically for this formula removes that risk and lets you check your own working in seconds.
This tool goes further than a basic single-purpose calculator by covering all six directions the formula can be solved in: the boiling point rise itself, molality, the van't Hoff factor, the ebullioscopic constant, an unknown solute's molar mass, and the mass of solute needed to hit a target boiling point. That range makes it useful for coursework, lab report calculations, and quick sanity checks on experimental ebullioscopy data alike.
Boiling Point Elevation Calculator FAQ and Quick Reference
Use ΔTb = i × Kb × m to find the boiling point rise directly. Rearrange to m = ΔTb / (iKb) to find molality, i = ΔTb / (Kb·m) to find the van't Hoff factor, or Kb = ΔTb / (i·m) to find an unknown solvent's ebullioscopic constant. To find an unknown solute's molar mass from lab data, use M = (i × Kb × mass of solute) / (ΔTb × kg of solvent).
This calculator is meant for study, homework checking, and general lab planning. Always confirm the van't Hoff factor for ionic solutes, double-check your solvent's Kb value against your course material, and verify your final numbers against your lab manual before submitting graded work.
Frequently Asked Questions
What is the formula for boiling point elevation?
ΔTb = i × Kb × m, where i is the van't Hoff factor, Kb is the solvent's ebullioscopic constant, and m is the molality of the solution.
What is Kb for water?
Water's ebullioscopic constant (Kb) is 0.512 °C·kg/mol, and its normal boiling point is 100 °C at standard atmospheric pressure.
How do you find molar mass from boiling point elevation?
Use M = (i × Kb × mass of solute) / (ΔTb × kg of solvent), after measuring the boiling point rise produced by a known mass of solute in a known mass of solvent.
Why does salt water boil at a higher temperature than pure water?
Dissolved salt ions lower the vapor pressure of the water, so a higher temperature is needed before the vapor pressure reaches atmospheric pressure and boiling begins.
Is boiling point elevation the same as freezing point depression?
They are related colligative properties from the same dissolved solute, but they use different constants (Kb vs Kf) and are usually different in size for the same solvent.
What van't Hoff factor should I use for NaCl?
Sodium chloride fully dissociates into Na+ and Cl- ions in water, so the ideal van't Hoff factor is i = 2.