Practical guides, how-tos and comparisons to get the most out of our free online tools.
Split a 120 ml batch at 1:1 and you get 60 ml of colour and 60 ml of developer; at 1:2 the same total becomes 40 and 80. The second number is the one that matters chemically, because the developer is what carries the hydrogen peroxide, and the peroxide is what lifts the existing pigment before the new one is deposited.
Antonio Delgado · October 19, 2026 · 4 min read · 2 sources
6,220 M⁻¹cm⁻¹ at 100 µM through 1 cm gives A = 0.622, and 23.9 % of the light emerges. Double the path length and A doubles to 1.244 — the scale is logarithmic, and that is where it breaks.
Lena Hoffmann · 4 min read
Fifty grams of salt in five litres is 0.171 mol/kg and buys +0.175 °C. Reaching a full degree would take 285 grams — most of a box.
Marco Bianchi · 3 min read
0.15 M of a salt that splits in two and 0.30 M of one that does not give the identical 7.64 atmospheres. Only the particle count reaches the membrane.
Sofia Nunes · 3 min read
Equal amounts of a weak acid and its base give pH = pKa exactly, at any concentration. Blood runs the same equation at a 20:1 ratio and lands on 7.40.
Sofia Nunes · 3 min read
24.80 mL of 0.1 M titrant into 25.00 mL of analyte gives 0.0992 M — or 0.0496 M if the analyte releases two protons. The stoichiometry is typed, not measured.
Lena Hoffmann · 3 min read
A 28 cm latex balloon nets 14 g of lift, so a kilogram takes 72 of them. At 2,000 metres the same balloon lifts 21 % less and you need 91.
Antonio Delgado · 3 min read
Ten items in three containers force four into one of them. 367 people force a shared birthday. Neither statement tells you which container or which pair.
Lena Hoffmann · 3 min read
A 50 mm lens at f/8 gives 10.47 m — or 15.68 m if you tighten the circle of confusion from 0.03 to 0.02 mm. Both settings are defensible and they differ by half.
Antonio Delgado · 4 min read
18 shots, 8 free throws and 3 turnovers in 36 minutes gives 29.3 % usage. Five players sharing a team's whole box score get exactly 20 % each — the number is a share, not a skill.
Aisha Karim · 4 min read
Four hours a day is 28 a week and 1,460 a year — 60.8 full days. Three hours is 45.6 days and seven is 106.5. The conversion is arithmetic; what it means is not.
Sofia Nunes · 3 min read
17 mod 5 is 2 everywhere. −17 mod 5 is −2 in JavaScript, C and Java, and 3 in Python, Ruby and a spreadsheet. Both are right, and the difference is a real bug.
Daniel Okonkwo · 3 min read