Maths - T1 W5 Moles, molarity and dilutions

What is the scientific definition of a mole?

The amount of a substance that contains the same number of elementary units (atoms, molecules, ions) as there are atoms in 0.012 kg of carbon-12

What is Avogadro's number?

6.022×10

23

particles per mole

What is the formula relating mass (m), number of moles (n), and relative molecular mass (M

r

​

)?

m=n×M

r

​

(or rearranged as n=

M

r

​

m

​

)

How many moles are in 360.36 g of glucose (M

r

​

=180.18)?

2 mol

What is molarity (M)?

The concentration of a solution expressed as the number of moles of solute per litre of solution

What does the N=C×V triangle represent for solutions?

Number of Moles (N) = Concentration (C) × Volume (V)

What happens to the total number of moles of a solute when a solution is diluted with extra solvent?

The total number of moles remains constant

What formula is used for dilution calculations involving molar solutions?

C

1

​

V

1

​

=C

2

​

V

2

​

(Initial Concentration × Initial Volume = Final Concentration × Final Volume)

What is the proper laboratory procedure when making a specific volume of solution at a given concentration?

Add the solute to a volumetric flask, dissolve it in a portion of the solvent, and then add additional solvent up to the calibration mark

Why should you not simply add the final volume of solvent to the solute?

Because adding the full volume of solvent on top of the solute will result in a final volume greater than desired and a lower concentration than intended

What instrument is used in scientific investigations to measure solution absorbance?

A spectrophotometer

What does the Beer-Lambert Law formula state?

Absorbance=εcl

In the Beer-Lambert Law (Absorbance=εcl), what do ε, c, and l represent?

ε is the molar extinction coefficient (M

−1

cm

−1

), c is concentration (M), and l is light path length (cm)

Why is a negative control (blank) used in spectrophotometry?

To zero the machine and subtract background absorbance from the solvent or container

What units does absorbance have?

Absorbance is dimensionless and has no units

What is the purpose of constructing a calibration curve?

To relate known concentrations of a substance to their measured absorbances so an unknown concentration can be determined

What part of a calibration curve should be used for accurate quantification?

The linear portion of the curve where absorbance is directly proportional to concentration

What equation describes a straight-line graph used in calibration curves?

y=mx+c (where m is the gradient and c is the y-intercept)

If the absorbance is 0.38, ε=6.22×10

3

M

−1

cm

−1

, and l=1 cm, what is the concentration (c)?

c=

εl

Absorbance

​

=

6.22×10

3

×1

0.38

​

≈6.11×10

−5

M

What amino acid extinction coefficient and wavelength are referenced in Application 3 of the Moles module?

Tyrosine has an extinction coefficient of 1405 M

−1

cm

−1

at 274 nm

How do you calculate the mass of NaCl needed to make 500 ml of a 100 mM solution (M

r

​

=58.44)?

V=0.5 l, C=0.1 mol/l, n=0.05 mol, so mass = 0.05×58.44=2.922 g

What is the formula for calculating the number of red blood cells or erythrocytes per mouse volume in Application 5?

Scaling up the measured count in a small sample volume (20 μl) to the total blood volume of the mouse (2.5 ml)

How do you calculate the remaining solid mass after evaporating a solution in a cylindrical tube (Application 6)?

Calculate total moles using concentration and volume (N=C×V), then multiply moles by the gram formula mass (M

r

​

)

How is the volume of a sphere or hemisphere calculated when evaluating tumor regression (Application 7)?

Using geometric radius formulas (V=

3

4

​

πr

3

for a sphere, or halved for a hemisphere) before and after treatment to find percentage reduction