How the stock solution calculator works
A stock solution is a concentrated solution you prepare once and then dilute for daily work. To make one from a solid, you need the mass of solute that gives the right number of moles in the final volume:
mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol)
The calculator converts mM, µM and nM to mol/L and mL or µL to liters before multiplying, then reports the mass in g, mg or µg, whichever reads best. Find the molecular weight (formula weight) on the reagent bottle or its certificate of analysis; for hydrated salts, use the weight of the exact form you are weighing.
Worked examples
Calcium chloride. How many grams of CaCl2 (110.98 g/mol) are needed for 250.0 mL of a 0.200 M solution? 0.200 mol/L × 0.2500 L × 110.98 g/mol = 5.55 g. This check-your-learning problem is from OpenStax Chemistry 2e, section 3.3, and it is the default in the calculator above.
Sodium chloride. OpenStax Example 3.17 asks for the mass of NaCl in 0.250 L of a 5.30 M solution: 5.30 mol/L × 0.250 L = 1.325 mol, which is 77.4 g.
Tris buffer. Promega’s guide to making a complex solution lists 121.1 g of Tris base for 1 L of 1 M Tris.
Making the stock solution step by step
- Calculate the mass with the calculator above and weigh it out.
- Dissolve the solid in less than the final volume of solvent.
- Transfer to a volumetric flask or graduated cylinder and bring the volume up to the mark.
- Mix well and label the container with the name, concentration and date.
Step 3 matters: the volume in the formula is the final solution volume, not the volume of solvent you start with.
From stock solution to working solution
Once the stock is made, dilute it with C1V1 = C2V2. Enter the stock concentration as C1 and your working concentration as C2 on the main dilution calculator, and it tells you how much stock to pipette. For example, a 200 mM stock diluted to 2.0 mM in 100 mL needs 1.0 mL of stock. If the working concentration is very low, plan a serial dilution so that no single step needs a tiny volume.
Stock solutions in other units
- Percent (w/v): 1% w/v is 1 g per 100 mL, so a 10% w/v stock needs 100 g made up to 1 L. The percent solution dilution calculator handles dilutions of percent stocks.
- Fold (X) buffers: a 10X stock is ten times the working strength. Diluting 100X to 1X in 50.0 mL needs 0.50 mL of stock; select X as the unit on the main calculator.
- mg/mL to molarity: divide the g/L value by the molecular weight. The main calculator does this automatically when C1 and C2 use different unit types and you enter the molecular weight.