15) Ca ^{2+} content was determined in the cheese extract by flame photometry. The analysis was performed by the addition method. The strength of the photocurrent during photometry of the analyzed sample ( *x* ) is equal to 17.0 μA. When adding the standard Ca ^{2+} solution to the same sample volume, the following data were obtained:

x+s , μg / cm _{ext.}^{3} |
x+ 10.0 |
x+ 20.0 |
x+ 30.0 |

I , µA |
35.0 | 52.0 | 70.0 |

Find the content of Ca ^{2+} in the cheese extract (µg/cm ^{3} ).

a) 10.0; b) 5.0; c) 3.0.

Decision:

Building a graph

The point of intersection of the straight line with the abscissa axis corresponds to the content of the analyte in the analyzed solution.

**Typical training tasks on Instrumental methods of analysis to prepare for the boundary control module 2.**

16) The excitation potential of the resonant level of the potassium atom is 1.6 eV. Calculate the wavelength (nm) of the resonance line in the atomic spectrum of potassium.

a) 500; b) 775; c) 650.

Decision:

∆E=h ** ^{.}** = h

**c/**

^{.}**λ**

h=4.135 ** ^{.}** 10

^{-15}EV/S

c = 3 ** ^{.}** 10

^{8}m/s = 3

**10**

^{.}^{17}nm/s

λ = h ** ^{.}** c/ ΔE = 4.135

**10**

^{.}^{-15}

**3**

^{.}**10**

^{.}^{17}/ 1.6 = 775 nm

17) When photometry of standard aqueous solutions of Na ^{+} salt on a flame photometer, the following data were obtained:

s (Na ^{+} ), μg / cm ^{3} |
10.0 | 20.0 |

I , µA |
30.5 | 50.5 |

When analyzing an aqueous sample, the photocurrent strength was 45.0 μA. Find the content of Na ^{+} in the water sample (µg/cm ^{3} ).

a) 8.5; b) 17; c) 0.17.

Decision:

Building a graph

C _{x} u003d 17 μg / cm ^{3}

18) When determining protein in barley, the optical density of the analyzed sample is 0.44 ( *l* = 2.0 cm). The optical density of a standard solution containing 0.50 mg/cm ^{3} of protein is 0.60 ( *l* = 3.0 cm). Calculate the protein content ^{of} barley (mg/cm3).

a) 1.1; b) 5.5; c) 0.55.

Decision:

We determine ε _{x} for the standard solution.

A = ε _{x} ** ^{.}** c

^{.}*l*

ε _{x} u003d 0.60 / 3 ** ^{.}** 0.50 = 0.4

Knowing ε _{x} , we determine C _{x}

A _{x} = ε _{x} C _{x} *l* _{x}

C _{x} u003d 0.44 / 2 ** ^{.}** 0.4 u003d 0.55 mg / cm

^{3}

19) The optical density of the water extract from the meat after the biuret reaction is 0.40. Find the light transmission of the analyzed extract (%).

a) 80.0; b) 39.8; c) 50.0.

Answer: 39.8%

Decision:

A u003d – lg TT u003d 10 ^{–} ^{A} u003d 10 ^{-0.40} u003d 10 ^{1} ** ^{.}** 10

^{0.60}= 0.398 percent 39.8%

23) Fe ^{3+} ions contained in apple juice reducers to Fe ^{2+} . During coulometric titration of the analyzed sample with electrogenerated Sn ^{2+ ions} at a constant current strength of 5.0 mA in an HCl solution, the electrolysis ended in 85 s. Calculate the mass (mg) of Fe ^{3+} in apple juice.

a) 0.25; b) 0.12; c) 0.17.

Answer: 0.25 mg

Decision:

m= jtM /nF = 5 ** ^{.}** 10

^{-3}

**85**

^{.}**55.85/1**

^{.}**96500 = 0.25 mg**

^{.} Fe ^{3+} + e Fe ^{2+}

24) To build a calibration graph, we obtained voltammograms of standard Pb ^{2+} solutions and measured the wave height:

c (Pb ^{2+} ) ^{.} 10 ^{6} , g / cm ^{3} |
0.50 | 1.00 | 1.50 | 2.00 |

h , mm |
4.0 | 8.0 | 12.0 | 16.0 |

To determine Pb ^{2+} in waste water, 50.00 ml of water were taken and the wave height was measured, it was 11.0 mm. Calculate the molar concentration of Pb ^{2+} in wastewater.

a) 6.73 ** ^{.}** 10

^{-6}; b) 6.73

**10**

^{.}^{-5}; c) 3.4

**10**

^{.}^{-6}.

Answer: 6.73 ** ^{.}** 10

^{-6}mol/l

Decision:

We build a graduated graph

Determine the unknown concentration

With _{Pb} 2+ = 1.4 ** ^{.}** 10

^{-6}g/ml

n( Pb ^{2+} ) = 1.4 ** ^{.}** 10

^{-6}/ 208 u003d 6.73

**10**

^{.}^{-9}mol

1 mn u003d 10 ^{-3}

Therefore, C ( Pb ^{2+} ) = 6.73 ** ^{.}** 10

^{-9}/ 10

^{-3}u003d 6.73

**10**

^{.}^{-6}mol/l

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