The SI base unit for amount of substance, defined as exactly 6.02214076 x 10^23 particles, is the mole.
Missing Word
mole
The mole is the SI unit for amount of substance. It lets chemists count particles by weighing, because one mole of any substance has a mass in grams equal to its formula mass in amu.
Category: The Mole
Question 2
Each formula unit of calcium chloride contains 2 chloride ions.
Missing Word
2
Calcium forms a Ca2+ ion and chloride forms a Cl- ion, so two chloride ions are needed to balance one calcium ion, giving CaCl2. It follows that one mole of calcium chloride contains 2 mol of chloride ions.
Category: The Mole
Question 3
The small numbers written after element symbols in a chemical formula, showing how many atoms of each element are present, are called subscripts.
Missing Word
subscripts
Subscripts tell how many atoms of an element appear in one formula unit or molecule; in H2O the subscript 2 means two hydrogen atoms. Each atomic mass is multiplied by its subscript when calculating a formula mass.
Category: Formula Mass
Question 4
The atomic mass unit is defined as exactly one-twelfth the mass of one atom of the mass-12 isotope of the element carbon.
Missing Word
carbon
Atomic masses are measured relative to the carbon-12 isotope, which is assigned a mass of exactly 12 amu. All other atomic masses are expressed on this same scale.
Category: Formula Mass
Question 5
The mass percentages of all the elements in a pure compound add up to 100 percent.
Missing Word
100
Percent composition divides each element's mass by the total mass of the compound, so the parts together must account for the whole, or 100 percent. Checking this sum is a quick way to catch arithmetic errors.
Category: Percent Composition
Question 6
The concentration unit found by dividing the amount of dissolved substance in moles by the volume of the solution in liters is called molarity.
Missing Word
molarity
Molarity, written M, equals moles of solute per liter of solution. A 1.0 M solution contains 1.0 mol of solute in each liter of the final solution.
Category: Molarity
Question 7
Solutions of precisely known concentration are prepared in a volumetric flask, which is filled to a single calibration mark on its narrow neck.
Missing Word
volumetric
A volumetric flask is calibrated to hold one exact volume when filled to its mark. The solute is dissolved in some water, and more water is added until the solution reaches the mark.
Category: Molarity
Question 8
A 1.0 M solution contains 1.0 mol of dissolved substance in each liter of the final solution.
Missing Word
liter
Molarity is defined per liter of solution, the total final volume, not per liter of solvent. A volume measured in milliliters must be divided by 1000 to convert it to liters before calculating molarity.
Category: Molarity
Question 9
The process of lowering a solution's concentration by adding more water is called dilution.
Missing Word
dilution
Dilution increases the volume of a solution while the amount of solute stays the same, so the concentration falls. Because moles are conserved, M1V1 = M2V2 relates the solution before and after.
Category: Dilution
Question 10
A chemist makes a dilute solution by adding water to a measured volume of a more concentrated stock solution.
Missing Word
concentrated
A concentrated solution holds a relatively large amount of solute per unit volume, and a stock solution is a concentrated one kept on hand for making dilutions. Adding water spreads the same solute through a larger volume, producing a more dilute solution.
Category: Dilution
Question 11
In a solution, the component present in the greatest amount, in which the other substances are dissolved, is the solvent.
Missing Word
solvent
The solvent is the major component of a solution, and the dissolved substances are spread evenly throughout it. Water is the most familiar example.
Category: Solutions
Question 12
A substance dissolved in a solution, usually present in a lesser amount than the dissolving medium, is a solute.
Missing Word
solute
A solute is the dissolved component of a solution, such as salt in salt water. Concentration units such as molarity and mass percentage describe how much of it is present.
Category: Solutions
Question 13
A solution in which water is the dissolving medium is described as aqueous.
Missing Word
aqueous
Aqueous means dissolved in water, and the symbol (aq) after a formula marks a substance in aqueous solution. Most solutions in a general chemistry lab are aqueous.
Category: Solutions
Question 14
Converting a mass percentage into a concentration based on solution volume requires the density of the solution, which relates its mass to its volume.
Missing Word
density
Mass percentage is based on solution mass, while molarity is based on solution volume. Density, the mass per unit volume, is the conversion factor that links the two.
Category: Concentration Units
Question 15
Blood glucose levels are commonly reported in milligrams per deciliter, a volume equal to 100 mL.
Missing Word
deciliter
A deciliter is one-tenth of a liter, or 100 mL. Reporting glucose in mg/dL is a mass-volume concentration; a typical normal fasting range is about 70 to 100 mg/dL.
Category: Concentration Units