Chemistry problems with AI: mass fraction, moles and calculations from an equation
Choose a problem type – the preview shows the calculation chain and a solution in the "Given – Solution – Answer" format. In the chat the AI works through your problem step by step, gives a hint or checks your solution.
Calculation chain
ω = m(solute) / m(solution)
Important: a common mistake is dividing by the mass of water instead of the mass of the whole solution.
Problem
20 g of salt was dissolved in 180 g of water. Find the mass fraction of the salt in the solution.
Given
m(salt) = 20 g
m(water) = 180 g
Find: ω(salt) – ?
Solution
1) m(solution) = m(salt) + m(water) = 20 + 180 = 200 g
2) ω = m(salt) / m(solution) = 20 / 200 = 0.1
3) 0.1 · 100 % = 10 %
Answer
ω(salt) = 10 %
Prompt sent to the chat: «Explain step by step how to solve a chemistry problem (grade 8, type: mass fraction of a substance in a solution). Problem: 20 g of salt was dissolved in 180 g of water. Find the mass fraction of the salt in the solution. Lay it out as in a notebook: Given, Solution, Answer; show the chain of conversions (mass → amount of substance → mass or volume) and explain each one. Before the answer, ask whether I can do the last calculation myself.»
You can attach a photo of the problem from your notebook. Free on the Starter plan (30 energy units).
Solve in chatMass fraction problems: work them out yourself
Mass fraction is the part of a solution’s mass made up by the dissolved substance. Enter your numbers: the calculator shows the formula and every step.
Composition of the solution
m(solution) = m(solute) + m(water) = 20 + 180 = 200 g
ω = m(solute) / m(solution) = 20 / 200 = 0.1
ω = 10 %
Formulas for chemistry problems
Eight formulas cover most calculation problems in grades 8–9. The molar mass is found from the periodic table: the sum of atomic masses, taking subscripts into account.
n = m / MAmount of substance, mol: mass divided by molar mass.
n = V / VmFor gases. Vm = 22.4 L/mol at standard conditions.
N = n · NₐNumber of particles. Avogadro constant Nₐ ≈ 6.02 · 10²³ mol⁻¹.
M = Mr g/molThe molar mass is numerically equal to the relative molecular mass.
ω = m(solute) / m(solution)Mass fraction of a substance in a solution, as a fraction or a percentage.
m(solution) = m(solute) + m(water)The mass of a solution is the sum of the masses of all its parts.
ω(E) = k · Ar / MrMass fraction of an element in a compound: k is the element’s subscript in the formula.
η = m(actual) / m(theor.)Yield: how much you got compared with what the equation predicts.
How to solve reaction equation problems: 5 steps
The classic layout: under the formulas go the amount, the molar mass and the mass. Example – burning 4 g of hydrogen.
| 2H₂ | + O₂ | → 2H₂O | |
|---|---|---|---|
| Coefficient | 2 | 1 | 2 |
| M, g/mol | 2 | 32 | 18 |
| m, g | 4 | – | ? |
| n, mol | 4 / 2 = 2 | – | 2 |
- 1
Write the equation and balance the coefficients. Without them the proportion between the substances will be wrong.
- 2
Find the amount of the known substance: n(H₂) = m / M = 4 g / 2 g/mol = 2 mol.
- 3
Use the coefficients to find the amount of the substance you need: hydrogen and water in the ratio 2 : 2, so n(H₂O) = 2 mol.
- 4
Convert to the quantity you need: m(H₂O) = n · M = 2 mol · 18 g/mol = 36 g. For a gas, V = n · Vm.
- 5
Check the units and common sense: the mass of the products cannot exceed the mass of all the starting substances.
Chemistry problems for grades 8, 9, 10 and 11: what comes up most often
Click a row – the problem type and the grade are set in the form at the top. You can paste your own problem into the "Your problem" field or send a photo.
Grade 8
Grade 9
Grade 10
Grade 11
Chemistry problems solved with explanations – free
Starter – 0 ₽
30 energy units. Hints, a step-by-step walkthrough and a check of your solution are text requests, included in the free plan, and so is a photo of the problem. For daily study there is Lite at ₽690 (150 energy) and Base at ₽990 (200 energy).
Frequently asked questions
How do I solve chemistry problems?
Write down what is given, find the formula or reaction equation you need, convert the mass into an amount of substance, do the calculation and convert the answer into the units you need. The AI follows these steps and explains every conversion.
How do I find the mass fraction of a substance in a solution?
Divide the mass of the dissolved substance by the mass of the whole solution: ω = m(solute) / m(solution). The mass of the solution is the substance plus the water. For example, 20 g of salt in 180 g of water gives 10 %.
How do I find the mass fraction of an element in a substance?
Multiply the element’s atomic mass by its subscript in the formula and divide by the relative molecular mass of the substance. For oxygen in water: 16 / 18 ≈ 88.9 %.
Which formulas do I need for chemistry problems in grade 8?
Most often n = m / M, n = V / Vm with Vm = 22.4 L/mol, N = n · Nₐ and the mass fraction formula. They are all collected in the "Formulas for chemistry problems" section of this page.
Will the AI check my solution to a chemistry problem?
Yes, in Check mode. Paste your solution: the AI checks the equation and the coefficients, the molar masses and the calculations, shows where the mistake is and explains how to fix it.
Can I solve organic chemistry problems?
Yes: hydrocarbon combustion, deriving a substance’s formula from mass fractions, calculations from equations with organic substances. The principle is the same – equation, amount of substance, proportion.