To make a 2-molar (2 M) solution of glucose, dissolve 360 grams of glucose in enough water to bring the final volume to 1 liter. The 180 dalton molecular mass means each mole weighs 180 grams, so 2 moles require 360 grams. Dissolve the powder first, then add water to the 1,000 mL mark, because volume must be final, not starting.
The arithmetic is trivial; the execution is where most people go wrong. A 2 M glucose solution is 360 g/L, roughly 20 times the concentration of normal blood glucose, and getting there requires attention to final volume, density, and dissolution behavior. This explainer covers why the number is 360, how molarity differs from the alternatives, what happens at the solubility edge, and the practical steps that separate a correct solution from a nearly correct one.
Why 2 M Glucose Solves to 360 Grams
Molarity is defined as moles of solute per liter of final solution. That last word, final, carries the entire calculation. Glucose's molecular mass of 180 daltons means one mole of glucose weighs 180 grams; a dalton is one-twelfth the mass of a carbon-12 atom, and 180 Da is about 180 g/mol. Two moles therefore weigh 360 grams, and dissolving 360 grams into a final volume of one liter gives 2 M (PubChem lists glucose at 180.16 g/mol, which rounds to 180 for this purpose).
The mechanism behind that definition matters. When you dissolve glucose, the molecules occupy space and disrupt water's hydrogen-bond network. The resulting solution has a volume slightly larger than the water you started with. If you add 360 g to 1 L of water, you get more than 1 L of solution, so the actual concentration lands below 2 M. Volumetric flasks solve this by marking a calibrated final volume line.
Molarity also drifts with temperature because solution volume expands and contracts while the moles stay fixed. Molality, by contrast, is moles per kilogram of solvent and does not change with temperature. For most bench work at room temperature the difference is under 1 percent, but for precise physical chemistry, the distinction is real.
One plain-language anchor: molarity counts molecules per volume of finished liquid, not grams per volume of water poured in.
Turning 360 Grams Into a Real Solution
A 2 M glucose solution contains about 360 g/L, which is 36 percent glucose by mass in water (roughly 0.36 g per mL). That is a viscous, dense liquid. Pure water at room temperature is about 1.00 g/mL; a 36 percent glucose solution is closer to 1.13 g/mL, so one liter of the solution weighs roughly 1,130 grams rather than 1,000.
That density gap is the counterintuitive detail most thin explainers skip. If you weigh 1,000 grams of the solution expecting a liter, you get about 885 mL and a concentration near 2.26 M. Weighing and volumetric measurement are not interchangeable at this concentration.
Dissolution also takes time. Glucose dissolves readily, but 360 g in a 1 L flask is a large load. Add water to about 70-80 percent of the final volume, stir or swirl until the solid is fully gone, then top up to the mark. Adding all the water first and then dumping in powder risks clumping and a slower dissolve.
Practical numbers worth carrying: 360 g for 1 L of 2 M; 180 g for 500 mL; 36 g for 100 mL. A 36 g/100 mL preparation is a convenient small batch and is close to the concentration of commercial glucose injections used in clinical settings (dextrose 50 percent is far more concentrated, so these are different products).
Molarity Versus Other Ways to Say 2 M
Chemists express the same glucose load in several units, and confusing them produces real errors. The table below reconciles the common alternatives at 2 M.
| Unit | Value for 2 M glucose | What it measures | Common confusion |
|---|---|---|---|
| Molarity (M) | 2.00 mol/L of solution | Moles per liter of final solution | Forgetting final volume |
| Mass concentration | 360 g/L | Grams per liter of solution | Assuming 360 g per liter of water |
| Mass percent | about 36% w/w | Grams glucose per 100 g solution | Mixing w/w with w/v |
| Molality (m) | about 2.26 mol/kg water | Moles per kilogram of solvent | Using it as if identical to M |
| Osmolarity | about 2.0 osmol/L | Dissolved particles per liter | Counting glucose as multiple particles |
The molality figure is higher than 2 because a liter of 2 M solution contains less than 1 kg of water; the solute displaces some of it. The osmolarity figure is 2.0 because glucose does not dissociate in water; one molecule stays one particle. Salts like sodium chloride split into two ions, which is why 1 M NaCl is roughly 2 osmol/L. This distinction drives why glucose solutions are described as isotonic, hypotonic, or hypertonic relative to body fluids, and why a 2 M glucose solution is strongly hypertonic (normal plasma osmolarity is about 0.29 osmol/L).
Where 2 M Glucose Gets Used, and What It Costs
A 2 M stock is a workhorse concentration in molecular biology. Many protocols call for glucose added to media or buffers at 0.1-0.5 percent, which corresponds to roughly 5.5-28 mM, so a 2 M stock is diluted 70- to 400-fold. Stock solutions save weighing small masses repeatedly and reduce weighing error.
In industry, glucose at high concentration feeds fermentation. A 2 M feed is about 360 g/L, and industrial fermentations often run glucose feed stocks in the 300-600 g/L range, though 2 M is on the upper end for handling convenience. Higher concentrations approach glucose's room-temperature solubility limit of roughly 900 g/L in water, above which crystallization becomes likely.
Cost context: laboratory-grade glucose (dextrose, anhydrous or monohydrate) is inexpensive, commonly in the range of USD 10-40 per kilogram from major suppliers; food-grade dextrose costs less. A 1 L batch of 2 M solution therefore uses about USD 4-15 of material. Prices vary by grade and vendor, so treat these as ballpark ranges rather than quotes.
Edge Cases: Hydrates, Solubility, and Sterility
The 180 g/mol figure assumes anhydrous glucose. If you use glucose monohydrate, the formula mass is about 198 g/mol because each molecule carries one water. Dissolving 360 g of the monohydrate gives fewer than 2 moles of glucose, so the solution is about 1.82 M rather than 2 M. Check the label: anhydrous versus monohydrate changes your weighing by about 10 percent.
Solubility is the hard ceiling. At 20-25 degrees Celsius, glucose dissolves to roughly 900 g/L in water. A 2 M solution at 360 g/L sits comfortably below that, so it is achievable at room temperature without heating. Pushing toward 4-5 M approaches saturation, and a chilled solution can crystallize.
Red-flag warnings worth posting on the bench: Do not add 360 g to a full liter of water and call it 2 M; the final volume will exceed 1 L and the concentration will be low. Do not confuse anhydrous and monohydrate forms; the mass difference is about 10 percent. Do not autoclave a concentrated glucose solution without checking compatibility; glucose can caramelize under prolonged heat, so sterile filtration is often preferred for stock solutions. Do not assume a 2 M glucose solution is safe for intravenous use; it is far more concentrated than clinical dextrose fluids and is not a substitute for medical products.
Misconceptions That Shift the Number
The most common error is treating "2 M" as "360 g in 1 L of water." Because volume is not additive, this yields roughly 1.8-1.9 M depending on starting volume, a 5-10 percent shortfall that matters in quantitative work.
A second misconception is that daltons and grams per mole are unrelated. They are numerically linked by Avogadro's number: 180 Da per molecule times about 6.02 × 10^23 molecules per mole gives about 180 g/mol. That bridge is why the molecular mass number directly gives the weighing number.
A third is assuming molarity equals osmolarity for all solutes. For glucose they match numerically, but for dissociating salts they do not. A laboratory that treats 2 M NaCl as 2 osmol/L will be off by nearly a factor of two.
A professional caveat: volumetric glassware is calibrated at a stated temperature, usually 20 degrees Celsius. Preparing a 2 M solution at 30 degrees Celsius and then using it at 4 degrees Celsius introduces a small concentration change from thermal contraction, typically well under 1 percent, but it is not zero. For analytical work, equilibrate and measure at the calibration temperature.
Practical Steps and Thresholds
Follow a sequence rather than a single weighing step.
Confirm the glucose form and its formula mass; use 180 g/mol for anhydrous and about 198 g/mol for monohydrate. Calculate the mass for your final volume: 360 g for 1 L, 180 g for 500 mL, 36 g for 100 mL of 2 M. Add water to roughly 70 percent of the final volume in a volumetric flask or graduated cylinder. Add the glucose and stir until fully dissolved; confirm no crystals remain at the bottom. Bring to the final volume mark, mix by inversion, and label with concentration, form used, and date. For sterile stock, filter through a 0.22-micron filter rather than autoclaving.
Thresholds to respect: stay below about 900 g/L to avoid crystallization at room temperature; keep final volume measurement within about 1 percent for reliable molarity; and note that solutions stored at 4 degrees Celsius may show slow crystallization near saturation, so 2 M is a practical storage concentration while 4-5 M is not.
Reconciling the thresholds: solubility (about 900 g/L) is the physical limit, the hydrate correction (about 10 percent) is an accuracy limit, and volumetric calibration (about 1 percent) is a precision limit. Each operates at a different scale, and a good preparation respects all three.
What the Calculation Cannot Tell You
The 360-gram answer assumes ideal behavior. At 2 M, glucose solutions are non-ideal: osmotic pressure, viscosity, and density deviate from simple proportional predictions. Activity coefficients differ from 1, which matters for precise thermodynamic work. For routine bench use these deviations are small enough to ignore; for physical chemistry they are not.
The calculation also says nothing about purity, moisture content, or degradation. An opened container of glucose can absorb water, effectively converting some anhydrous material to hydrate and lowering the delivered molarity. Weighing quickly and storing sealed reduces this drift.
Finally, the number is a recipe, not a guarantee of fitness for purpose. Whether 2 M glucose suits a given experiment, fermentation, or buffer depends on osmotic tolerance, dilution schedule, and sterility requirements that the arithmetic cannot address.