Chemistry calculators and simulators
Molarity, pH, titration, stoichiometry and rate law calculators, plus a periodic table, an equation balancer and galvanic cell and Bohr model simulators.
43 tools
These are bench calculations: the arithmetic between deciding what you need and having it in a tube. Most of it is the same handful of relationships applied under time pressure with a balance in one hand, converting moles to mass, concentration to volume, acid to base.
Everything here converts units internally, which matters more than it sounds. A large share of failed preparations are not chemistry mistakes at all but a factor of a thousand in the wrong place.
Nearly all of it passes through the mole, and the mole passes through molar mass, which makes the formula you type the most load-bearing thing on the page. Anhydrous copper sulfate is 159.60 g/mol and the pentahydrate 249.68, so treating one as the other leaves you weighing about 36 percent too little.
Not all of it is bench work. The simulators and visualisers cover the parts of chemistry whose answer is a picture rather than a number: where the electrons sit in an atom, what a titration curve does either side of its equivalence point, how a cell’s voltage falls as it discharges. The two that are most often run together are worth separating deliberately, because they answer different questions: the free energy change decides whether a reaction can go at all and the activation energy decides how long it takes, so a large negative free energy change says nothing on its own about how fast the reaction will be.
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Molarity, molality and percent are not interchangeable
Molarity is moles per litre of solution, so it changes with temperature as the solution expands. Molality is moles per kilogram of solvent, which does not. For bench work at room temperature the difference is negligible and molarity wins on convenience, but for anything involving boiling points, freezing points or a wide temperature range, molality is the one that behaves.
Percent is worse, because it means three different things. Weight per volume, weight per weight and volume per volume all get written as percent, and a 10 percent solution can differ by a factor of two depending on which was meant. Say which you mean, every time.
Weight per volume at least converts cleanly: 1 percent w/v is 1 g per 100 mL, which is 10 g/L, which is 10 mg/mL, and in dilute aqueous solution 1 mg/L is taken as 1 ppm. Reaching molarity needs the molar mass too, which is why saline at 0.9 percent w/v comes to 154 mM once you divide 9 g/L by 58.44.
Why buffers resist and where they stop
A buffer holds a reservoir of both a weak acid and its conjugate base, so added protons are absorbed by the base and added hydroxide by the acid. The pH follows the ratio of the two, not their absolute amounts, which is why diluting a buffer barely shifts its pH while it retains any capacity at all.
Capacity is the limit people forget. A buffer is only useful within about one pH unit either side of its pKa, and once one component is consumed the pH moves as freely as if the buffer were never there. The ratio sets the pH; the total concentration sets the reserve, so a 100 mM buffer absorbs roughly ten times the added acid a 10 mM buffer does at the same pH. Choose a buffer whose pKa is near your target pH.
Temperature sits underneath all of it. pH plus pOH equals 14 only at 25 °C, because the sum is pKw and pKw is temperature dependent. Tris is the notorious case: its pKa shifts by roughly 0.03 pH units per degree Celsius, so a Tris buffer adjusted to pH 8.0 on the bench is nearer pH 8.5 in a cold room. Set the pH at the temperature the experiment will run at, and record which that was.
Stoichiometry, and where a clean factor of two comes from
A result wrong by exactly two, or exactly three, is almost never an arithmetic slip. It is a mole ratio. Hydrochloric acid against sodium hydroxide is one to one, but sulfuric acid supplies two protons per molecule, so the ratio is two and an answer that ignores it is out by precisely that factor. Read the ratio off the balanced equation rather than assuming it.
The same reasoning governs yield, which is built from the limiting reagent: the one that runs out first, not necessarily the one present in the smallest mass. Two checks apply at the far end. A yield above 100 percent is not a good result, it is a wet or impure product, since residual solvent adds mass without adding product. And yields multiply across a synthesis, so five steps at 80 percent each leave 33 percent overall.
What a bottle label actually tells you
Concentrated acids are labelled by weight percent, which you cannot pipette from directly. The molarity needs the density as well: concentrated hydrochloric acid at about 37 percent w/w and 1.18 g/mL comes to roughly 12 M, and concentrated sulfuric acid at about 98 percent w/w and 1.84 g/mL to roughly 18 M. Any dilution from a stock bottle starts with that conversion, not with the percent on the label.
Some reagents drift in storage and cannot be trusted from the mass you weighed. Sodium hydroxide pellets absorb both water and carbon dioxide from the air, which is why analytical work standardises a hydroxide titrant against a primary standard rather than assuming its nominal concentration.
There is also an order of operations no calculation will remind you of. Dissolve a solid in less than the target volume and top up to the mark afterwards, because molarity is defined per litre of final solution. With concentrated acids, add the acid to the water, since the mixing is strongly exothermic and a large volume of water absorbs that heat where a small volume of acid cannot.
Common questions
How do I convert a percent solution to molarity?
For weight per volume, first turn the percent into grams per litre by multiplying by 10, since 1 percent w/v is 10 g/L. Then divide by the molar mass to get moles per litre. Physiological saline is the standard worked example: 0.9 percent w/v is 9 g/L, and 9 divided by the 58.44 g/mol of sodium chloride gives 0.154 M. The conversion is impossible without a molar mass, which is why percent is used for mixtures and proteins where no single molar mass exists.
Why does the pH of my solution drift after I make it up?
Three causes account for most of it. Dissolved carbon dioxide from the air acidifies an unbuffered or weakly buffered solution over hours. Temperature changes the pKa of the buffer itself, and for Tris that is about 0.03 pH units per degree Celsius, so moving a solution between a bench and a cold room shifts it measurably. Beyond that, a buffer near the end of its capacity has little reserve left, so a small amount of added acid or base moves the pH much further than the same amount did when the buffer was fresh.