Streptomyces:Other Bits/Useful Molecular and Chemical Equations

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Other Bits - Useful Molecular and Chemical Equations

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Useful Molecular and Chemical Equations

Formula Weight & Molecular Weight

Formula weight (FW) and molecular weight (MW) are calculated by summing the atomic weights (AW measured in atomic mass units, amu) of the individual atoms.

e.g. where:
C = 12.01amu
H = 1.00amu
Na = 35.45amu
Cl = 22.99amu

Chemical Name Chemical Formula Weight

Benzene C6H6 MW = 6*12.01 + 6*1.00 = 78.06amu
Sodium Chloride NaCl FW = 22.99 + 35.45 = 58.44amu

The difference between formula weight and molecular weight depends on the compound. It is correct to refer to a compound such as Benzene having a molecular weight or formula weight. It is incorrect to refer to sodium chloride having a molecular weight as NaCl exists as an ionic compound (Na+ Cl-) not as a molecular compound. In this case it is more precise to refer to sodium chloride’s formula weight.

Avogadro’s Number & mole

1 mole of atoms / molecules has a mass equal to the atomic / molecular weight in grams.

e.g. 1 mole (1mol) NaCl is the number of molecules in 58.44g of NaCl. (1mol NaCl = 58.44g)

Avogadro’s number is the number of atoms / molecules in 1 mole of any substance, which is equal to 6.02214x1023.

n=\frac {m}{FW}

Molarity – Molar Concentration

Molarity is the number of moles of solute per litre of solution.

e.g. 6 molar (6M) HCl is equal to 6 moles (6mol) of HCl per litre (L). (6M HCl = 6mol/L)

n = Number of moles
m = Mass in grams (g)
FW = Formula weight
M = Molarity in mol/litre (mol/L)
V = Volume in litres (L)
M=\frac {n}{V}

Based on the previous two equations:

m = MVFW M=\frac {m}{VFW}

Primer Calculations

Primers are dissolved in sterile distilled water (sdH2O) to a concentration of 500pmol. Use one of the following to determine what volume of sdH2O to use:

\frac {\mu g*10^{6}}{500*MW}=\mu L \frac {nmol}{0.5}=\mu L \frac {pmol}{500}=\mu L

Weight / mole Percentage

The percentage weight of an element in a compound is calculated using the atomic weight and formula weight.

Chemical Name Chemical Formula Weight

Hydrochloric acid HCl FW = 1.00 + 35.45 = 36.45amu

\frac {AW}{FW}*100=%weight

Percentage weight of Cl in HCl:

Cl=\frac {35.45}{36.45}=97.26%

Similarly, mole percentage is a ratio.

x = Number of atoms of the element
T = Total number of atoms in the compound

\frac {x}{T}*100=%mole

Percentage mole of Cl in HCl:

Cl=\frac {1}{2}*100=50%

Density and Specific Gravity

Density is the mass of a substance per volume.

D = Density (g/cc)
m = Mass in grams (g)
v = Volume in cubic centimetres (cc)
SG = Specific Gravity

Specific gravity is a unitless ratio, so for all purposes; SG ≡ D. Cubic centimetres are equivalent to millilitres; cc ≡ mL.

D=\frac {m}{v} SG=\frac {D_1}{D_2}

D2 = Density of H2O @ 4°C = 1.00g/cc

Molarity, Specific Gravity and Percentage Composition

Calculating Molarity from specific gravity and percentage composition:

Chemical Name Formula weight Percentage Composition Specific Gravity

Hydrochloric acid 36.45amu 37% 1.18

Percentage composition means xg of pure compound per 100g of solution, i.e. 37g/100ml = 37%. To calculate the molarity, the mass of pure compound is needed; however the solution’s specific gravity needs to be taken into account, and the volume; which we’ll take to be 1L.

m = SG * v * %composition M=\frac {m}{VFW}

Where 1L = 1000cc For HCl:

m=\frac {1.18*1000*37}{100}=436.6g


M=\frac {436.6}{1*36.45}=11.97mol/L

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